Display device, module, and electronic device

ABSTRACT

A high-definition liquid crystal display device is provided. A liquid crystal display device with a high aperture ratio is provided. A liquid crystal display device with a high contrast ratio and display quality is provided. A liquid crystal display device capable of being driven at a low voltage is provided. The display device includes, between a pair of substrates, a pixel electrode, a first common electrode, a second common electrode, and a liquid crystal layer. The pixel electrode and the first common electrode are positioned between the liquid crystal layer and one of the substrates. The second common electrode is positioned between the liquid crystal layer and the other substrate. The same potential is supplied to the first common electrode and the second common electrode. The first common electrode includes a portion overlapping with the second common electrode between the display regions of two adjacent subpixels that exhibit different colors. At least one of the pixel electrode and the first common electrode includes a portion that does not overlap with the second common electrode in the display region of the subpixel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/455,244, filed Mar. 10, 2017, now allowed, which claims the benefitof foreign priority applications filed in Japan as Serial No.2016-050824 on Mar. 15, 2016, and Serial No. 2016-101543 on May 20,2016, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to a liquid crystaldisplay device, a module, and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention include a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a memorydevice, an electronic device, a lighting device, an input device (suchas a touch sensor), an input/output device (such as a touch panel), amethod for driving any of them, and a method for manufacturing any ofthem.

BACKGROUND ART

Transistors used for most flat panel displays typified by liquid crystaldisplay devices and light-emitting display devices are formed usingsilicon semiconductors such as amorphous silicon, single-crystalsilicon, and polycrystalline silicon provided over glass substrates. Thetransistors using such a silicon semiconductor are utilized forintegrated circuits (ICs) and the like.

In recent years, techniques in which a metal oxide that exhibitssemiconductor characteristics is used instead of a silicon semiconductorin a transistor have attracted attention. Note that in thisspecification, a metal oxide that exhibits semiconductor characteristicsis referred to as an oxide semiconductor. For example, Patent Documents1 and 2 disclose techniques for the fabrication of a transistor usingzinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and theuse of the transistor as a switching element or the like in a pixel of adisplay device.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

An object of one embodiment of the present invention is to provide ahigh-definition liquid crystal display device. Another object of oneembodiment of the present invention is to provide a liquid crystaldisplay device with a high aperture ratio. Another object of oneembodiment of the present invention is to provide a liquid crystaldisplay device with a high contrast ratio and display quality. Anotherobject of one embodiment of the present invention is to provide a liquidcrystal display device capable of being driven at a low voltage. Anotherobject of one embodiment of the present invention is to provide a liquidcrystal display device with low power consumption. Another object of oneembodiment of the present invention is to provide a highly reliableliquid crystal display device. Another object of one embodiment of thepresent invention is to provide a novel liquid crystal display device.

Note that the description of these objects does not exclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects can bederived from the description of the specification, the drawings, theclaims, and the like.

A display device of one embodiment of the present invention includes,between a pair of substrates, a pixel electrode, a first commonelectrode, a second common electrode, and a liquid crystal layer. Thepixel electrode and the first common electrode are positioned betweenthe liquid crystal layer and one of the substrates. The second commonelectrode is positioned between the liquid crystal layer and the othersubstrate. The same potential is supplied to the first common electrodeand the second common electrode. The first common electrode includes aportion overlapping with the second common electrode between the displayregions of two adjacent subpixels that exhibit different colors. Atleast one of the pixel electrode and the first common electrode includesa portion that does not overlap with the second common electrode in thedisplay region of the subpixel.

The second common electrode preferably includes an opening in thedisplay region of a subpixel. When the thickness of the liquid crystallayer is denoted by d, the width of the opening is preferably greaterthan or equal to d/6 and narrower than the width of the subpixel. Whenthe thickness of the liquid crystal layer is denoted by d, the distancebetween the openings is preferably greater than or equal to d and lessthan or equal to 2.5d. The thickness d of the liquid crystal layer ispreferably greater than or equal to 1 μm and less than or equal to 3 μm.

The first common electrode may be electrically connected to the secondcommon electrode. Alternatively, a potential may be independentlysupplied to the first common electrode and the second common electrode.For example, the first common electrode and the second common electrodemay be electrically connected to different power source lines.

A liquid crystal included in the liquid crystal layer preferably has anegative dielectric anisotropy.

The display device preferably includes a transistor that includes anoxide semiconductor in its channel formation region. The transistor iselectrically connected to the pixel electrode. The semiconductor layerof the transistor preferably includes, for example, indium, zinc, andone of aluminum, gallium, yttrium, and tin.

Preferably, the display device includes a scan line and a signal line,the direction in which the scan line extends intersects with thedirection in which the signal line extends, and a plurality of subpixelsexhibiting the same color are aligned in a direction intersecting withthe direction in which the signal line extends.

One embodiment of the present invention is a module that includes thedisplay device according to any of the above, where a connector such asa flexible printed circuit (FPC) board or a tape carrier package (TCP)is connected or an IC is implemented with a method such as a chip onglass (COG) method or a chip on film (COF) method.

In one embodiment of the present invention, the above structures may beapplied to an input/output device (e.g., a touch panel) instead of thedisplay device.

One embodiment of the present invention is an electronic deviceincluding the aforementioned module and at least one of an antenna, abattery, a housing, a camera, a speaker, a microphone, and a controlbutton.

According to one embodiment of the present invention, a high-definitionliquid crystal display device can be provided. According to anotherembodiment of the present invention, a liquid crystal display devicewith a high aperture ratio can be provided. According to anotherembodiment of the present invention, a liquid crystal display devicewith a high contrast ratio and display quality can be provided.According to another embodiment of the present invention, a liquidcrystal display device capable of being driven at a low voltage can beprovided. According to another embodiment of the present invention, aliquid crystal display device with low power consumption can beprovided. According to another embodiment of the present invention, ahighly reliable liquid crystal display device can be provided. Accordingto another embodiment of the present invention, a novel liquid crystaldisplay device can be provided.

Note that the description of these effects does not exclude theexistence of other effects. In one embodiment of the present invention,there is no need to achieve all the effects. Other effects can bederived from the description of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D are cross-sectional views illustrating examples of aliquid crystal element;

FIGS. 2A to 2C are top views illustrating layouts of a second commonelectrode;

FIG. 3A is a perspective view illustrating an example of a displaydevice, and FIGS. 3B and 3C are top views illustrating examples ofsubpixels;

FIGS. 4A and 4B are cross-sectional views illustrating examples of adisplay device;

FIGS. 5A and 5B illustrate arrangement and structure examples of pixels;

FIG. 6 is a cross-sectional view illustrating an example of a displaydevice;

FIG. 7 is a cross-sectional view illustrating an example of a displaydevice;

FIGS. 8A to 8D are cross-sectional views illustrating examples of adisplay device;

FIGS. 9A and 9B are cross-sectional views illustrating examples of adisplay device;

FIG. 10 is a cross-sectional view illustrating an example of a displaydevice;

FIGS. 11A and 11B are perspective views illustrating an example of atouch panel;

FIG. 12 is a cross-sectional view illustrating an example of a touchpanel;

FIGS. 13A and 13B illustrate an example of an input device and anexample of a method for driving the input device;

FIGS. 14A and 14B are perspective views illustrating an example of atouch panel;

FIG. 15 is a cross-sectional view illustrating an example of a touchpanel;

FIG. 16 is a cross-sectional view illustrating an example of a touchpanel;

FIGS. 17A to 17D are top views illustrating examples of an input device;

FIGS. 18A to 18E are top views illustrating examples of an input device;

FIG. 19 is a cross-sectional view illustrating an example of a touchpanel;

FIGS. 20A and 20B illustrate examples of a sensor element and a pixel;

FIGS. 21A to 21E illustrate operation examples of a sensor element and apixel;

FIGS. 22A to 22C are top views illustrating examples of a sensor elementand a pixel;

FIG. 23 is a block diagram illustrating an example of a touch panelmodule;

FIGS. 24A to 24C illustrate examples of a touch panel module;

FIGS. 25A1, 25A2, 25B1, 25B2, 25C1, and 25C2 are cross-sectional viewsillustrating examples of a transistor;

FIGS. 26A1 to 26A3, 26B1, and 26B2 are cross-sectional viewsillustrating examples of a transistor;

FIGS. 27A1 to 27A3, 27B1, 27B2, 27C1, and 27C2 are cross-sectional viewsillustrating examples of a transistor;

FIGS. 28A to 28C are a top view and cross-sectional views illustratingan example of a transistor;

FIGS. 29A to 29C are a top view and cross-sectional views illustratingan example of a transistor;

FIGS. 30A to 30C are a top view and cross-sectional views illustratingan example of a transistor;

FIGS. 31A and 31B are a top view and a cross-sectional view illustratingan example of a transistor;

FIGS. 32A and 32B are a top view and a cross-sectional view illustratingan example of a transistor;

FIGS. 33A to 33C are a top view and cross-sectional views illustratingan example of a transistor;

FIGS. 34A to 34C are a top view and cross-sectional views illustratingan example of a transistor;

FIGS. 35A to 35C are a top view and cross-sectional views illustratingan example of a transistor;

FIG. 36 illustrates an example of a touch panel module;

FIGS. 37A to 37H illustrate examples of electronic devices;

FIGS. 38A and 38B illustrate examples of electronic devices;

FIGS. 39A and 39B illustrate pixel layouts in Example 1;

FIGS. 40A and 40B show alignment simulation results in Example 1;

FIGS. 41A and 41B show alignment simulation results in Example 1;

FIGS. 42A and 42B show alignment simulation results in Example 1;

FIGS. 43A and 43B show alignment simulation results in Example 1;

FIG. 44 shows simulation results in Example 1;

FIG. 45A is a photograph showing a display result of a display device inExample 1, and FIGS. 45B and 45C are optical micrographs of a displayportion;

FIGS. 46A and 46B show simulation results in Example 2; and

FIGS. 47A and 47B show simulation results in Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails can be modified in various ways without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the description inthe following embodiments.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and the description of suchportions is not repeated. Furthermore, the same hatching pattern isapplied to portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

The position, size, range, or the like of each structure illustrated indrawings is not accurately represented in some cases for easyunderstanding. Therefore, the disclosed invention is not necessarilylimited to the position, size, range, or the like disclosed in thedrawings.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film”. Also,the term “insulating film” can be changed into the term “insulatinglayer”.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention will be described with reference to FIGS. 1A to 24C.

The display device of one embodiment of the present invention includes apixel electrode, a first common electrode, a second common electrode,and a liquid crystal layer. Each of the pixel electrode and the firstcommon electrode faces the second common electrode with the liquidcrystal layer therebetween in the thickness direction of the displaydevice. The same potential is supplied to the first common electrode andthe second common electrode. The first common electrode includes aportion overlapping with the second common electrode between the displayregions of two adjacent subpixels that exhibit different colors. Atleast one of the pixel electrode and the first common electrode includesa portion that does not overlap with the second common electrode in thedisplay region of the subpixel.

The display device includes a plurality of pixels and has a function ofdisplaying images.

A pixel includes a plurality of subpixels. For example, a subpixelexhibiting a red color, a subpixel exhibiting a green color, and asubpixel exhibiting a blue color form one pixel, and thus full-colordisplay can be achieved in a display portion. Note that the colorexhibited by subpixels is not limited to red, green, and blue. Forexample, a subpixel exhibiting white, yellow, magenta, cyan, or the likemay be used for a pixel. Note that in this specification and the like, asubpixel is simply referred to as a pixel in some cases.

Examples of a driving method of a liquid crystal display device includethe following: frame inversion driving, where the positive and negativeelectrodes are inverted (i.e., the polarities of the signals areinverted) frame by frame; gate-line inversion driving, where thepositive and negative electrodes are inverted row by row; source-lineinversion driving, where the positive and negative electrodes areinverted column by column; and dot-line inversion driving, where thepositive and negative electrodes are inverted column by column and rowby row. The burn-in of the images can be prevented by inverting thepolarities of the signals using these driving methods. The source-lineinversion driving is preferably used in terms of power consumption.

An increase in the definition of a liquid crystal display device reducesthe width (distance) between pixels and the width (distance) betweensubpixels. Hence, for example, when the source-line inversion driving isemployed for a display device using a liquid crystal element with ahorizontal electric field mode, a horizontal electric field is generatedbetween adjacent subpixels, which might cause alignment defects ofliquid crystals and light leakage to an adjacent subpixel. The lightleakage reduces the display quality of the display device. A decrease indisplay quality can be reduced when a light-blocking layer or the likecovers a portion that is prone to light leakage; however, this mightreduce the aperture ratio.

Thus, in one embodiment of the present invention, between displayregions of two subpixels that exhibit different colors, the liquidcrystal layer is interposed between a pair of electrodes (the firstcommon electrode and the second common electrode) supplied with the samepotential. This prevents a horizontal electric field from beinggenerated between the adjacent two subpixels. As a result, the alignmentdefects of liquid crystals can be prevented to reduce light leakage,increasing the contrast ratio of the display device.

In one embodiment of the present invention, at least one of the pixelelectrode and the first common electrode includes a portion that doesnot overlap with the second common electrode in the display region ofthe subpixel. As a result, the driving voltage of a liquid crystalelement is unlikely to increase even when the second common electrode isprovided.

<1-1. Structure Example 1 of Display Device>

FIGS. 1A to 1D illustrate cross-sectional views of the display device ofone embodiment of the present invention.

The display device illustrated in FIG. 1A includes a substrate 119 a, asubstrate 119 b, a pixel electrode 111 a, a pixel electrode 111 b, afirst common electrode 112, a liquid crystal layer 113, a second commonelectrode 244, and an insulating layer 220.

The display device illustrated in FIG. 1A includes display regions 68 aand 68 b. The display regions 68 a and 68 b are display regions ofsubpixels that exhibit different colors (i.e., openings in thesubpixels).

The pixel electrodes 111 a and 111 b and the first common electrode 112are positioned between the liquid crystal layer 113 and the substrate119 a. The second common electrode 244 is positioned between the liquidcrystal layer 113 and the substrate 119 b. The same potential issupplied to the first common electrode 112 and the second commonelectrode 244.

In the display device illustrated in FIG. 1A, the first common electrode112 is over the substrate 119 a, the insulating layer 220 is over thefirst common electrode 112, and the island-shaped pixel electrodes 111 aand 111 b are over the insulating layer 220. The pixel electrode isprovided in each subpixel. In the display region, the pixel electrodehas an opening or an aperture (also referred to as a slit or the like).

The display device illustrated in FIG. 1B is different from that in FIG.1A in the stacking order of the pixel electrode and the first commonelectrode.

In the display device illustrated in FIG. 1B, the island-shaped pixelelectrodes 111 a and 111 b are over the substrate 119 a, the insulatinglayer 220 is over the pixel electrodes 111 a and 111 b, and the firstcommon electrode 112 is over the insulating layer 220. In the displayregion, the first common electrode 112 has an opening or an aperture(also referred to as a slit or the like).

In each of the display regions 68 a and 68 b, a voltage can be appliedbetween the pixel electrode and the first common electrode 112 (seearrows in FIGS. 1A and 1B). In contrast, between the display regions 68a and 68 b, the liquid crystal layer 113 is interposed between the firstcommon electrode 112 and the second common electrode 244 supplied withthe same potential (a constant potential, a common potential). Thecommon potential supplied to the electrode on the substrate 119 b sideprevents the electric field from spreading from the pixel electrode tothe electrodes in adjacent subpixels. As a result, the alignment defectsof liquid crystals can be prevented to reduce light leakage, increasingthe contrast ratio of the display device.

In FIG. 1A, the first common electrode 112 has a portion that does notoverlap with the second common electrode 244 in each of the displayregions 68 a and 68 b. In FIG. 1B, the pixel electrode 111 a has aportion that does not overlap with the second common electrode 244 inthe display region 68 a, and the pixel electrode 111 b has a portionthat does not overlap with the second common electrode 244 in thedisplay region 68 b. As compared with the case where the second commonelectrode 244 is provided in the entire display region of the subpixel,an increase in the driving voltage of the liquid crystal element can bereduced when the second common electrode 244 is partly provided.

In FIGS. 1A and 1B, L1 denotes the length of the display region of thesubpixel where the second common electrode 244 is not provided, and L2denotes the length of the second common electrode 244 that is providedacross two subpixels. In FIG. 1A, the thickness of the liquid crystallayer 113 between the pixel electrode and the second common electrode244 is denoted by d. In FIG. 1B, the thickness of the liquid crystallayer 113 between the first common electrode 112 and the second commonelectrode 244 is denoted by d. The thickness d of the liquid crystallayer refers to the thickness of the liquid crystal layer 113 betweenthe second common electrode 244 and one of the pixel electrode and thefirst common electrode 112 that is closer to the second common electrode244 in the thickness direction of the liquid crystal layer 113. Thethickness d of the liquid crystal layer can also be referred to as acell gap or the minimum distance between the second common electrode 244and one of the pixel electrode and the first common electrode 112.

FIGS. 2A to 2C illustrate layout examples of the second common electrode244.

In the examples shown here, one pixel is composed of three subpixels ofa red subpixel (R), a green subpixel (G), and a blue subpixel (B). Aregion other than a display region 68 in a subpixel is denoted by anon-display region 66.

FIG. 2A shows an example in which the second common electrode 244 has anopening. The opening is positioned in at least part of the displayregion 68. The opening may be extended in the non-display region 66.

In FIG. 2A, the length L1 is equal to the width of the opening. In otherwords, the length L1 is the length of a short side of the opening, thelength of the opening in the direction where subpixels exhibitingdifferent colors are aligned, or the like.

In FIG. 2A, the length L2 is equal to the distance between openings. Inother words, the length L2 is the distance or the like of openings inthe direction where subpixels exhibiting different colors are aligned.

FIG. 2B shows an example in which a plurality of second commonelectrodes 244 are provided in a stripe pattern. The direction in whichthe second common electrodes 244 are aligned intersects with thedirection in which subpixels exhibiting the same color are aligned.

One of the second common electrodes 244 is provided across two adjacentsubpixels exhibiting different colors. For example, the second commonelectrode 244 a is provided across the red subpixel (R) and the greensubpixel (G).

In FIG. 2B, the length L1 is equal to the distance between two adjacentsecond common electrodes.

In FIG. 2B, the length L2 is equal to the width of the second commonelectrode. In other words, the length L2 is the length of a short sideof the second common electrode, the length of the second commonelectrode in the direction where subpixels exhibiting different colorsare aligned, or the like.

Note that the second common electrode 244 illustrated in FIG. 2B can beregarded as a comb-like electrode. In that case, the second commonelectrodes 244 a, 244 b, and 244 c are connected to one another in aportion not illustrated in FIG. 2B. The length L1 can be the distancebetween teeth whereas the length L2 can be the width of a tooth.

FIG. 2C shows an example in which the opening in the second commonelectrode 244 is across two adjacent subpixels exhibiting the samecolor. The opening may be positioned in the display region 68 in aplurality of subpixels exhibiting the same color.

The second common electrode 244 is preferably provided in a larger areato have a lower resistance. For example, the resistance of the secondcommon electrode 244 can be lower in the structure of FIG. 2A than inthe structures of FIGS. 2B and 2C.

The following description is made on the case where the second commonelectrode 244 in FIGS. 1A to 1D has the layout illustrated in FIG. 2A.In FIGS. 1A to 1D, the second common electrode 244 has an opening in thedisplay region 68. In FIGS. 1B to 1D, the first common electrode 112 hasan opening in the display region 68.

FIGS. 1C and 1D are different from FIG. 1B in the shape of the secondcommon electrode 244.

As illustrated in FIG. 1C, the first common electrode 112 may also havea portion that does not overlap with the second common electrode 244 ineach of the display regions 68 a and 68 b.

In FIG. 1B, the width of the opening in the first common electrode 112is equal to the width of the opening in the second common electrode 244.

In FIG. 1C, the width of the opening in the second common electrode 244is greater than the width of the opening in the first common electrode112.

In FIG. 1D, the width of the opening in the second common electrode 244is narrower than the width of the opening in the first common electrode112.

Seen from the direction perpendicular to the thickness of the liquidcrystal layer 113, the length from an end portion of the opening in thesecond common electrode 244 to an end portion of the opening in thefirst common electrode 112 is denoted by L3 in FIG. 1C and L4 in FIG.1D.

When the second common electrode 244 is provided in a larger area in asubpixel, the spread of the electric field from the pixel electrode tothe electrodes in adjacent subpixels can be more reduced. In otherwords, light leakage can be reduced with a shorter length L1 or a longerlength L2. Also, a decrease in the length L3 shown in FIG. 1C can reducelight leakage, and an increase in the length L4 shown in FIG. 1D canreduce light leakage.

When the second common electrode 244 is provided in a smaller area inthe display region of a subpixel, an increase in the driving voltage ofthe liquid crystal element due to the second common electrode 244 can bereduced. In other words, an increase in the driving voltage of theliquid crystal element can be reduced with a longer length L1 or ashorter length L2. Also, an increase in the length L3 shown in FIG. 1Ccan reduce an increase in the driving voltage of the liquid crystalelement, and a decrease in the length L4 shown in FIG. 1D can reduce anincrease in the driving voltage of the liquid crystal element.

The liquid crystal layer with a smaller thickness d can increase theeffect of the second common electrode 244 and reduce the generation ofthe horizontal electric field between two subpixels. A reduction in thethickness d of the liquid crystal layer results in an increase in thelength L1 (a decrease in the length L2). As a result, both light leakageand an increase in driving voltage can be prevented.

In view of the above, when the thickness of the liquid crystal layer isdenoted by d, the length L1 is preferably greater than or equal to d/6,more preferably greater than or equal to d/2.

When the thickness of the liquid crystal layer is denoted by d, thelength L2 is preferably greater than or equal to d and less than orequal to 2.5d, more preferably greater than or equal to 1.2d and lessthan or equal to 2.4d. The condition of the length L2 affects thecontrast ratio of the display device. The condition of the length L1affects the driving voltage of the display device. Hence, the conditionof the length L2, which affects display quality, is preferably givenpriority in the fabrication of the display device.

The thickness d of the liquid crystal layer is preferably greater thanor equal to 1 μm and less than or equal to 3 μm, more preferably greaterthan or equal to 1.5 μm and less than or equal to 3 μm.

According to one embodiment of the present invention, light leakagebetween adjacent subpixels can be prevented and therefore, the distancebetween the subpixels can be reduced. This increases the aperture ratioof the subpixel, increases the definition of the display device,improves the display quality of the display device, and reduces anincrease in driving voltage. Furthermore, a higher aperture ratioincreases the light extraction efficiency. As a result, the powerconsumption of the display device can be reduced.

<1-2. Structure Example 2 of Display Device>

FIG. 3A and FIG. 4A illustrate an example of the display device. FIG. 3Ais a perspective view of a display device 100A, and FIG. 4A is across-sectional view of the display device 100A. For clarity, componentssuch as a polarizer 130 are not drawn in FIG. 3A. FIG. 3A illustrates asubstrate 61 with a dotted line.

The display device 100A includes a display portion 62 and a drivercircuit portion 64. An FPC 72 and an IC 73 are implemented on thedisplay device 100A.

The display portion 62 includes a plurality of pixels and has a functionof displaying images.

The display device 100A may include one or both of a scan line drivercircuit and a signal line driver circuit. The display device 100A mayinclude none of the scan line driver circuit and the signal line drivercircuit. When the display device 100A includes a sensor such as a touchsensor, the display device 100A may include a sensor driver circuit. Inthis embodiment, the driver circuit portion 64 is exemplified asincluding the scan line driver circuit. The scan line driver circuit hasa function of outputting scan signals to the scan lines included in thedisplay portion 62.

In the display device 100A, the IC 73 is mounted on a substrate 51 by aCOG method or the like. The IC 73 includes, for example, any one or moreof a signal line driver circuit, a scan line driver circuit, and asensor driver circuit.

The FPC 72 is electrically connected to the display device 100A. The IC73 and the driver circuit portion 64 are supplied with signals or powerfrom the outside through the FPC 72. Furthermore, signals can be outputto the outside from the IC 73 through the FPC 72.

An IC may be mounted on the FPC 72. For example, an IC including any oneor more of a signal line driver circuit, a scan line driver circuit, anda sensor driver circuit may be mounted on the FPC 72.

A wiring 65 supplies signals and power to the display portion 62 and thedriver circuit portion 64. The signals and power are input to the wiring65 from the outside through the FPC 72, or from the IC 73.

FIGS. 3B and 3C are top views of subpixels included in the displaydevice 100A.

FIG. 4A is a cross-sectional view including the display portion 62, thedriver circuit portion 64, and the wiring 65. FIG. 4A includes across-sectional view along dashed-dotted line X1-X2 in FIG. 3B. In FIG.4A and the subsequent cross-sectional views of the display device, thedisplay portion 62 includes the display region 68 in a subpixel and thenon-display region 66 around the display region 68.

FIG. 3B is a top view seen from the first common electrode 112 side andillustrates a layered structure from a gate 223 to the first commonelectrode 112 in the subpixel (see FIG. 4A). In FIG. 3B, the displayregion 68 in the subpixel is outlined in a bold dotted line. FIG. 3C isa top view of the layered structure of FIG. 3B except for the firstcommon electrode 112.

The display device 100A is an example of a transmissive liquid crystaldisplay device that includes a liquid crystal element with a horizontalelectric field mode.

As illustrated in FIG. 4A, the display device 100A includes thesubstrate 51, a transistor 201, a transistor 206, a liquid crystalelement 40, an auxiliary wiring 139, an alignment film 133 a, analignment film 133 b, a connection portion 204, an adhesive layer 141, acoloring layer 131, a light-blocking layer 132, an overcoat 121, thesubstrate 61, the polarizer 130, and the like.

The liquid crystal element 40 is provided in the display region 68. Theliquid crystal element 40 is a liquid crystal element with fringe fieldswitching (FFS) mode.

The liquid crystal element 40 includes a pixel electrode 111, the firstcommon electrode 112, the second common electrode 244, and the liquidcrystal layer 113. The alignment of the liquid crystal layer 113 can becontrolled with the electric field generated between the pixel electrode111 and the first common electrode 112. The liquid crystal layer 113 ispositioned between the alignment films 133 a and 133 b.

In a connection portion 69, the second common electrode 244 iselectrically connected to a conductive layer provided on the substrate51 side. Hence, a potential can be supplied to the second commonelectrode 244 through the FPC 72. This is preferable because there is noneed of connecting the FPC and the like on the substrate 61 side and thestructure of the display device can be more simplified.

The connection portion 69 may be part of the display portion 62.Alternatively, the connection portion 69 may be outside of the displayportion 62, and for example, may be provided between the display portion62 and the driver circuit portion 64.

The first common electrode 112 and the second common electrode 244 canbe supplied with the same potential. For example, when the first commonelectrode 112 and a conductive layer 284 are electrically connected toeach other or made of a film (the same film), the second commonelectrode 244 is electrically connected to the first common electrode112.

Note that the second common electrode 244 is not necessarilyelectrically connected to the first common electrode 112. In the casewhere the first common electrode 112 and the second common electrode 244are electrically connected to different power source lines, the samepotential is supplied to the two power source lines so that the firstcommon electrode 112 and the second common electrode 244 can be suppliedwith the same potential.

In the connection portion 69, a conductive layer 281 is connected to aconductive layer 282, the conductive layer 282 is connected to aconductive layer 283, the conductive layer 283 is connected to theconductive layer 284, the conductive layer 284 is connected to aconnector 243, and the connector 243 is connected to the second commonelectrode 244. The conductive layer 281, the conductive layer 282, andthe conductive layer 283 can be formed using the same material and thesame fabrication step as those used in the gate 223 of the transistor,the gate 221 of the transistor, and the conductive layers 222 a and 222b, respectively. Fabricating the conductive layers in the connectionportion 69 in such a manner, i.e., using the same materials and the sameprocesses as the conductive layers used in the display portion 62 andthe driver circuit portion 64, is preferable because the number ofprocess steps is not increased.

As the connector 243, a conductive particle can be used, for example Aparticle of an organic resin, silica, or the like coated with a metalmaterial can be used as the conductive particle. Nickel or gold ispreferably used as the metal material because contact resistance can bedecreased. A use of a particle coated with layers of two or more kindsof metal materials, such as a particle coated with nickel and furtherwith gold, is also preferable. A material capable of elastic deformationor plastic deformation is preferably used as the connector 243. Asillustrated in FIG. 4A and the like, the conductive particle has a shapethat is vertically crushed in some cases. With the crushed shape, thecontact area between the connector 243 and a conductive layerelectrically connected to the connector 243 can be increased, therebyreducing contact resistance and reducing issues such as disconnection.

The connector 243 is preferably provided so as to be covered with theadhesive layer 141. For example, the connector 243 may be dispersedwithin the adhesive layer 141 before the curing thereof.

In FIG. 4A, the pixel electrode 111 is electrically connected to alow-resistance region 231 b through the conductive layer 222 b.

As illustrated in FIG. 4B, the pixel electrode 111 may be directlyconnected to the low-resistance region 231 b. In that case, asemiconductor layer (a channel region 231 a and the low-resistanceregion 231 b) preferably contains a material transmitting visible light,such as an oxide semiconductor. This allows the pixel electrode 111 andthe connection portion of the transistor to be provided in the displayregion 68, increasing the aperture ratio of the subpixel and thedefinition of the display device. Note that the low-resistance region231 b may be electrically connected to the conductive layer 222 b. Theconductive layer 222 b can serve as an auxiliary electrode of thelow-resistance region 231 b. The transistor does not necessarily includethe conductive layer 222 b.

The first common electrode 112 may have a top-surface shape (alsoreferred to as a planar shape) that has a comb-like shape or atop-surface shape that is provided with a slit. FIGS. 3B and 3C and FIG.4A illustrate an example where one opening is provided in the firstcommon electrode 112 in the display region 68 of one subpixel. As thedisplay device has higher definition, the area of the display region 68in one subpixel becomes smaller. Thus, the number of openings providedin the first common electrode 112 is not limited to more than one; oneopening can be provided. That is, in a display device with highdefinition, the area of the pixel (subpixel) is small; therefore, anadequate electric field for the alignment of liquid crystals over theentire display region of the subpixel can be generated, even when thereis only one opening in the first common electrode 112.

The insulating layer 220 is provided between the pixel electrode 111 andthe first common electrode 112. The pixel electrode 111 includes aportion that overlaps with the first common electrode 112 with theinsulating layer 220 provided therebetween. Furthermore, the firstcommon electrode 112 is not placed above the pixel electrode 111 in someareas of a region where the pixel electrode 111 and the coloring layer131 overlap. The auxiliary wiring 139 is provided over the first commonelectrode 112. The resistivity of the auxiliary wiring 139 is preferablylower than that of the first common electrode 112. By providing anauxiliary wiring that is electrically connected to the common electrode,a drop in voltage due to the resistance of the common electrode can beinhibited. In addition, when a layered structure of a conductive layerincluding a metal oxide and a conductive layer including a metal isused, these conductive layers are formed preferably by a patterningtechnique using a half tone mask, thereby simplifying the fabricationprocess.

The auxiliary wiring 139 is a film with smaller resistance than thefirst common electrode 112. For example, the auxiliary wiring 139 can beformed to have a single-layer structure or a layered structure using anyof metal materials such as molybdenum, titanium, chromium, tantalum,tungsten, aluminum, copper, silver, neodymium, and scandium, and analloy material containing any of these elements.

The auxiliary wiring 139 is preferably provided in a position thatoverlaps with the light-blocking layer 132 and the like, so that theauxiliary wiring 139 is not seen by the user of the display device.

An alignment film is preferably provided in contact with the liquidcrystal layer 113. The alignment film can control the alignment of theliquid crystal layer 113. In the display device 100A, the alignment film133 a is positioned between the first common electrode 112 (or theinsulating layer 220) and the liquid crystal layer 113, and thealignment film 133 b is positioned between the second common electrode244 (or the overcoat 121) and the liquid crystal layer 113.

The liquid crystal material is classified into a positive liquid crystalmaterial with a positive dielectric anisotropy (Δε) and a negativeliquid crystal material with a negative dielectric anisotropy. Both ofthe materials can be used in one embodiment of the present invention,and an optimal liquid crystal material can be selected according to theemployed mode and design.

In one embodiment of the present invention, a negative liquid crystalmaterial is preferably used. The negative liquid crystal is lessaffected by a flexoelectric effect, which is attributed to thepolarization of liquid crystal molecules, and thus the polarity ofvoltage applied to the liquid crystal layer makes little difference intransmittance. This prevents flickering from being recognized by theuser of the display device. The flexoelectric effect is a phenomenon inwhich polarization is induced by the distortion of orientation, andmainly depends on the shape of a molecule. The negative liquid crystalmaterial is less likely to experience the deformation such as spreadingand bending.

Note that the liquid crystal element 40 is an element using an FFS modehere; however, one embodiment of the present invention is not limitedthereto, and a liquid crystal element using any of a variety of modescan be used. For example, a liquid crystal element using a verticalalignment (VA) mode, a twisted nematic (TN) mode, an in-plane switching(IPS) mode, an axially symmetric aligned micro-cell (ASM) mode, anoptically compensated birefringence (OCB) mode, a ferroelectric liquidcrystal (FLC) mode, or an antiferroelectric liquid crystal (AFLC) modecan be used.

Furthermore, the display device 100A may be a normally black liquidcrystal display device, for example, a transmissive liquid crystaldisplay device using a vertical alignment (VA) mode. Examples of thevertical alignment mode include a multi-domain vertical alignment (MVA)mode, a patterned vertical alignment (PVA) mode, and an advanced superview (ASV) mode.

The liquid crystal element is an element that controls transmission andnon-transmission of light by optical modulation action of the liquidcrystal. The optical modulation action of a liquid crystal is controlledby an electric field applied to the liquid crystal (including ahorizontal electric field, a vertical electric field, and an obliqueelectric field). As the liquid crystal used for the liquid crystalelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal(PDLC), a ferroelectric liquid crystal, an anti-ferroelectric liquidcrystal, or the like can be used. Such a liquid crystal materialexhibits a cholesteric phase, a smectic phase, a cubic phase, a chiralnematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which 5 wt. % or more of a chiralmaterial is mixed is used for the liquid crystal layer 113 in order toimprove the temperature range. The liquid crystal composition thatincludes a liquid crystal exhibiting a blue phase and a chiral materialhas a short response time and exhibits optical isotropy, which makes thealignment process unnecessary. In addition, the liquid crystalcomposition that includes a liquid crystal exhibiting a blue phase and achiral material has little viewing angle dependence. In addition, sincean alignment film does not need to be provided and rubbing treatment isunnecessary, electrostatic discharge damage caused by the rubbingtreatment can be prevented and defects or damage of the liquid crystaldisplay device in the manufacturing process can be reduced.

As the display device 100A is a transmissive liquid crystal displaydevice, a conductive material that transmits visible light is used forboth the pixel electrode 111 and the first common electrode 112. In thecase where the second common electrode 244 is positioned in the displayregion 68, a conductive material that transmits visible light is alsoused for the second common electrode 244.

For example, a material containing one or more of indium (In), zinc(Zn), and tin (Sn) is preferably used for the conductive material thattransmits visible light. Specifically, indium oxide, indium tin oxide(ITO), indium zinc oxide, indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, indium tin oxidecontaining silicon oxide (ITSO), zinc oxide, and zinc oxide containinggallium are given, for example Note that a film including graphene canbe used as well. The film including graphene can be formed, for example,by reducing a film containing graphene oxide.

Preferably, at least one of the pixel electrode 111 and the first commonelectrode 112 includes an oxide conductive layer. The oxide conductivelayer preferably includes one or more metal elements that are includedin the semiconductor layer of the transistor 206. For example, the pixelelectrode 111 preferably contains indium and is further preferably anIn-M-Zn oxide (M is Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn, or Hf) film.Similarly, the first common electrode 112 preferably contains indium andis further preferably an In-M-Zn oxide film.

At least one of the pixel electrode 111 and the first common electrode112 may be formed with an oxide semiconductor. When two or more layersconstituting the display device are formed using oxide semiconductorscontaining the same metal element, the same manufacturing equipment(e.g., film-formation equipment or processing equipment) can be used intwo or more steps; manufacturing cost can thus be reduced.

An oxide semiconductor is a semiconductor material whose resistance canbe controlled by oxygen vacancies in the film of the semiconductormaterial and/or the concentration of impurities such as hydrogen orwater in the film of the semiconductor material. Thus, the resistivityof the oxide conductive layer can be controlled by selecting betweentreatment for increasing oxygen vacancies and/or impurity concentrationon the oxide semiconductor layer, or treatment for reducing oxygenvacancies and/or impurity concentration on the oxide semiconductorlayer.

Note that such an oxide conductive layer formed using an oxidesemiconductor layer can be referred to as an oxide semiconductor layerhaving a high carrier density and a low resistance, an oxidesemiconductor layer having conductivity, or an oxide semiconductor layerhaving high conductivity.

In addition, the manufacturing cost can be reduced by forming the oxidesemiconductor layer and the oxide conductive layer using the same metalelement. For example, the manufacturing cost can be reduced by using ametal oxide target with the same metal composition. By using the metaloxide target with the same metal composition, an etching gas or anetchant used in the processing of the oxide semiconductor layer can alsobe used for processing of the oxide conductive layer. Note that evenwhen the oxide semiconductor layer and the oxide conductive layer havethe same metal elements, their composition of the metal elements aredifferent in some cases. For example, metal elements in the film candesorb during the fabrication process of the display device, whichresults in a different metal composition.

For example, when a silicon nitride film containing hydrogen is used forthe insulating layer 220, and an oxide semiconductor is used for thepixel electrode 111, the conductivity of the oxide semiconductor can beincreased by the hydrogen that is supplied from the insulating layer220.

The transistor 206 is provided in the non-display region 66.

The transistor 206 includes the gate 221, the gate 223, an insulatinglayer 211, an insulating layer 213, and a semiconductor layer (thechannel region 231 a and a pair of low-resistance regions 231 b). Theresistivity of the low-resistance region 231 b is lower than that of thechannel region 231 a. In this embodiment, the case in which an oxidesemiconductor layer is used as the semiconductor layer is described asan example. The oxide semiconductor layer preferably includes indium andis further preferably an In-M-Zn oxide (M is Al, Ti, Ga, Ge, Y, Zr, La,Ce, Nd, Sn, or Hf) film. The details of the oxide semiconductor layer isdescribed later.

The gate 221 and the channel region 231 a overlap with the insulatinglayer 213 positioned therebetween. The gate 223 and the channel region231 a overlap with the insulating layer 211 positioned therebetween. Theinsulating layers 211 and 213 serve as gate insulating layers. Throughopenings provided in the insulating layers 212 and 214, the conductivelayer 222 a is connected to one of the low-resistance regions 231 b andthe conductive layer 222 b is connected to the other of thelow-resistance regions 231 b.

The transistor 206 illustrated in FIG. 4A is a transistor includinggates above and below the channel.

In a contact area Q1 illustrated in FIG. 3C, the gates 221 and 223 areelectrically connected. A transistor that that has two gates that areelectrically connected to each other can have a higher field-effectmobility and thus have higher on-state current than the othertransistors. Consequently, a circuit capable of high-speed operation canbe obtained. Furthermore, the area occupied by a circuit portion can bereduced. The use of the transistor having a high on-state current canreduce signal delay in wirings and can reduce display unevenness even ina display device in which the number of wirings is increased because ofan increase in size or resolution. In addition, the use of such aconfiguration allows the fabrication of a highly reliable transistor.

In a contact area Q2 illustrated in FIG. 3C, the conductive layer 222 bis connected to the pixel electrode 111.

In other words, in FIGS. 3B and 3C, one conductive layer serves as ascan line 228 and the gate 223. One of the gates 221 and 223 that hasthe lower resistance of the two is preferably the conductive layer thatalso serves as the scan line.

In other words, in FIGS. 3B and 3C, one conductive layer serves as asignal line 229 and the conductive layer 222 a.

The gates 221 and 223 can each include a single layer of one of a metalmaterial and an oxide conductor, or stacked layers of both a metalmaterial and an oxide conductor. For example, one of the gates 221 and223 may include an oxide conductor, and the other of the gates 221 and223 may include a metal material.

The transistor 206 can be formed to include the oxide semiconductorlayer as a semiconductor layer, and include the oxide conductive layeras at least one of the gates 221 and 223. In this case, the oxidesemiconductor layer and the oxide conductive layer are preferably formedusing an oxide semiconductor.

When a conductive layer blocking visible light is used for the gate 223,light from a backlight can be prevented from entering the channel region231 a; this makes the transistor more reliable.

The transistor 206 is covered by the insulating layers 212 and 214 andinsulating layers 215 and 216. Note that the insulating layers 212, 214,and 216 can be considered as the component of the transistor 206. Thetransistor is preferably covered by an insulating layer that reduces thediffusion of an impurity to the semiconductor constituting thetransistor. The insulating layer 215 serves as a planarization layer.

Each of the insulating layers 211 and 213 preferably includes an excessoxygen region. When the gate insulating layer includes the excess oxygenregion, excess oxygen can be supplied into the channel region 231 a. Ahighly reliable transistor can be provided since oxygen vacancies thatare potentially formed in the channel region 231 a can be filled withexcess oxygen.

The insulating layer 212 preferably includes nitrogen or hydrogen. Whenthe insulating layer 212 and the low-resistance region 231 b are incontact with each other, nitrogen or hydrogen in the insulating layer212 is added into the low-resistance region 231 b. The carrier densityof the low-resistance region 231 b becomes high when nitrogen orhydrogen is added. Alternatively, when the insulating layer 214 includesnitrogen or hydrogen and the insulating layer 212 transmits the nitrogenor hydrogen, the nitrogen or hydrogen can be added into thelow-resistance region 231 b.

In the display device 100A, the coloring layer 131 and thelight-blocking layer 132 are provided closer to the substrate 61 thanthe liquid crystal layer 113. The coloring layer 131 is positioned in aregion that at least overlaps with the display region 68 of a subpixel.In the non-display region 66 of a pixel (subpixel), the light-blockinglayer 132 is provided. The light-blocking layer 132 overlaps with atleast a part of the transistor 206.

The overcoat 121 is preferably provided between the coloring layer 131or the light-blocking layer 132, and the liquid crystal layer 113. Theovercoat 121 can reduce the diffusion of an impurity contained in thecoloring layer 131, the light-blocking layer 132, and the like into theliquid crystal layer 113. In FIG. 4A, the second common electrode 244 isprovided between the overcoat 121 and the alignment film 133 b.

The substrates 51 and 61 are bonded to each other by the adhesive layer141. The liquid crystal layer 113 is encapsulated in a region that issurrounded by the substrates 51 and 61, and the adhesive layer 141.

When the display device 100A functions as a transmissive liquid crystaldisplay device, two polarizers are positioned in a way that the displayportion 62 is sandwiched by the two polarizers. FIG. 4A illustrates thepolarizer 130 on the substrate 61 side. Light 45 from a backlightprovided on the outside of the polarizer on the substrate 51 side entersthe display device 100A through the polarizer. In this case, the opticalmodulation of the light can be controlled by controlling the alignmentof the liquid crystal layer 113 with a voltage supplied between thepixel electrode 111 and the first common electrode 112. That is, theintensity of light that is ejected through the polarizer 130 can becontrolled. Furthermore, the coloring layer 131 absorbs light ofwavelengths other than a specific wavelength range from the incidentlight. As a result, the ejected light exhibits red, blue, or greencolors, for example.

In addition to the polarizer, a circular polarizer can be used, forexample. An example of a circular polarizer include a polarizer which isformed by stacking a linear polarizer and a quarter-wave retardationfilm. The circular polarizer can reduce the viewing angle dependence ofthe display quality of the display device.

The driver circuit portion 64 includes the transistor 201.

The transistor 201 includes the gate 221, the gate 223, the insulatinglayer 211, the insulating layer 213, the semiconductor layer (thechannel region 231 a and a pair of low-resistance regions 231 b), theconductive layer 222 a, and the conductive layer 222 b. One of theconductive layers 222 a and 222 b serves as a source, and the otherserves as a drain. The conductive layer 222 a is electrically connectedto one of the low-resistance regions 231 b and the conductive layer 222b is connected to the other of the low-resistance regions 231 b.

In the connection portion 204, the wiring 65 and a conductive layer 251are connected to each other, and the conductive layer 251 and aconnector 242 are connected to each other. That is, in the connectionportion 204, the wiring 65 is electrically connected to the FPC 72through the conductive layer 251 and the connector 242. By employingthis configuration, signals and power can be supplied from the FPC 72 tothe wiring 65.

The wiring 65 can be formed with the same material and the samefabrication step as those used in the conductive layers 222 a and 222 bthat are included in the transistor 206. The conductive layer 251 can beformed with the same material and the same fabrication step as thoseused in the pixel electrode 111 that is included in the liquid crystalelement 40. Fabricating the conductive layers constituting theconnection portion 204 in such a manner, i.e., using the same materialsand the same fabrication processes as those used in the conductivelayers composing the display portion 62 and the driver circuit portion64, is preferable because the number of process steps is not increased.

The transistors 201 and 206 may or may not have the same structure. Thatis, the transistors included in the driver circuit portion 64 and thetransistors included in the display portion 62 may or may not have thesame structure. In addition, the driver circuit portion 64 may have aplurality of transistors with different structures, and the displayportion 62 may have a plurality of transistors with differentstructures. For example, a transistor including two gates that areelectrically connected to each other is preferably used for one or moreof a shift register circuit, a buffer circuit, and a protection circuitincluded in a scan line driver circuit.

The pixel arrangement examples are shown in FIGS. 5A and 5B. FIGS. 5Aand 5B show examples in which one pixel is composed of a red subpixel R,a green subpixel G, and a blue subpixel B. In FIGS. 5A and 5B, aplurality of scan lines 81 extend in the x direction, and a plurality ofsignal lines 82 extend in the y direction. The scan lines 81 and thesignal lines 82 intersect with each other.

As shown by the dashed-two-dotted line in FIG. 5A, a subpixel includesthe transistor 206, a capacitor 34, and the liquid crystal element 40. Agate of the transistor 206 is electrically connected to the scan line81. One of a source and a drain of the transistor 206 is electricallyconnected to the signal line 82, and the other is electrically connectedto one electrode of the capacitor 34 and one electrode of the liquidcrystal element 40. The other electrode of the capacitor 34 and theother electrode of the liquid crystal element 40 are each supplied witha constant potential.

FIGS. 5A and 5B show examples where the source-line inversion driving isadopted. Signals A1 and A2 are signals with the same polarity. SignalsB1 and B2 are signals with the same polarity. Signals A1 and B1 aresignals with different polarities. Signals A2 and B2 are signals withdifferent polarities.

As the definition of the display device becomes higher, the distancebetween the subpixels become shorter. Thus, as shown in the frameoutlined in a dashed-dotted line in FIG. 5A, in the subpixel where thesignal A1 is input, the liquid crystal is easily affected by potentialsin both the signal A1 and the signal B1, in the vicinities of the signalline 82 where the signal B1 is input. This can make the liquid crystalmore prone to alignment defects.

In FIG. 5A, the direction in which a plurality of subpixels exhibitingthe same color are aligned is the y direction, and is substantiallyparallel to the direction where the signal lines 82 extend. As shown inthe frame outlined in the dashed-dotted line in FIG. 5A, subpixelsexhibiting different colors are adjacent to each other, with the longersides of the subpixels facing each other.

In FIG. 5B, the direction in which a plurality of subpixels exhibitingthe same color are aligned is the x direction, and intersects with thedirection where the signal lines 82 extend. As shown in the frameoutlined in a dashed-dotted line in FIG. 5B, subpixels exhibiting thesame color are adjacent to each other, with the shorter sides of thesubpixels facing each other.

When the side of the subpixel that is substantially parallel to thedirection in which the signal lines 82 extend is the shorter side of thesubpixel as illustrated in FIG. 5B, the region where the liquid crystalis more prone to alignment defects can be made narrower, compared withthe case (illustrated in FIG. 5A) where the longer side of the subpixelis substantially parallel to the direction in which the signal lines 82extend. When the region where the liquid crystal is more prone toalignment defects is positioned between subpixels exhibiting the samecolor as illustrated in FIG. 5B, display defects are less easilyrecognized by a user of the display device when compared with the case(see FIG. 5A) where the region is positioned between subpixelsexhibiting different colors. In one embodiment of the present invention,the direction in which the plurality of subpixels exhibiting the samecolor are arranged preferably intersects with the direction in which thesignal lines 82 extend.

In the display device of one embodiment of the present invention, thesecond common electrode 244 contributes to preventing the alignmentdefects of liquid crystals. Hence, one embodiment of the presentinvention can employ the structure illustrated in FIG. 5A, in which thedirection in which a plurality of subpixels exhibiting different colorsare aligned intersects with the direction where the signal lines 82extend.

FIG. 6 shows a cross-sectional view of a display device 100B. Note thatthe perspective view of the display device 100B is similar to that ofthe display device 100A illustrated in FIG. 3A; thus, the descriptionthereof is omitted.

The display device 100A shows an example where the transistor includestwo gates; in the display device 100B, the transistors 201 and 206 eachinclude only the gate 221. In addition, the display device 100B includesa spacer 117. Components of the display device 100B that are similar tothose of the display device 100A are not described in detail.

The transistors 201 and 206 are provided over the insulating layer 211.The insulating layer 211 serves as a base film. The transistor 206includes the gate 221, the insulating layer 213, and the semiconductorlayer (the channel region 231 a and a pair of low-resistance regions 231b). Through openings provided in the insulating layers 212 and 214, theconductive layer 222 a is connected to one of the low-resistance regions231 b and the conductive layer 222 b is connected to the other of thelow-resistance regions 231 b. The insulating layer 215 serves as aplanarization layer.

In the connection portion 69, the conductive layer 281 is connected tothe conductive layer 282, the conductive layer 282 is connected to theconductive layer 283, the conductive layer 283 is connected to theconnector 243, and the connector 243 is connected to the second commonelectrode 244. The conductive layer 281 and the conductive layer 282 canbe formed using the same material and the same fabrication step as thoseused in the gate 221, and the conductive layers 222 a and 222 b,respectively. Fabricating the conductive layers in the connectionportion 69 in such a manner, i.e., using the same materials and the sameprocesses as the conductive layers used in the display portion 62 andthe driver circuit portion 64, is preferable because the number ofprocess steps is not increased.

The spacer 117 has a function of keeping the distance between thesubstrate 51 and the substrate 61 greater than or equal to a certaindistance.

In the example shown in FIG. 6 , the bottom surface of the spacer 117 isin contact with the overcoat 121; however, one embodiment of the presentinvention is not limited thereto. The spacer 117 may be provided on thesubstrate 51 side, or the substrate 61 side.

In the example shown in FIG. 6 , the alignment films 133 a and 133 b arenot in contact with each other in a region where the alignment films 133a and 133 b overlap with the spacer 117; however, the alignment films133 a and 133 b may be in contact with each other. Furthermore, thespacer 117 provided over one substrate may be, but is not necessarily,in contact with a structure provided over the other substrate. Forexample, the liquid crystal layer 113 may be positioned between thespacer 117 and the structure.

A particulate spacer may be used as the spacer 117. As the particulatespacer, materials such as silica can be used. Spacer is preferably madeof a material with elasticity, such as a resin or rubber. In this case,the particulate spacer may take a shape that is vertically crushed.

Next, the details of the materials that can be used for components ofthe display device of this embodiment and the like are described. Notethat description on the components already described is omitted in somecases. The materials described below can be used as appropriate in thedisplay device, the touch panel, and the components thereof describedlater.

<<Substrates 51 and 61>>

There are no large limitations on the material of the substrate used inthe display device of one embodiment of the present invention; a varietyof substrates can be used. For example, a glass substrate, a quartzsubstrate, a sapphire substrate, a semiconductor substrate, a ceramicsubstrate, a metal substrate, or a plastic substrate can be used.

The weight and thickness of the display device can be reduced by using athin substrate. Furthermore, a flexible display device can be obtainedby using a substrate that is thin enough to have flexibility.

The display device of one embodiment of the present invention isfabricated by forming a transistor and the like over the fabricationsubstrate, then transferring the transistor and the like on anothersubstrate. The use of the fabrication substrate enables the following: aformation of a transistor with favorable characteristics; a formation ofa transistor with low power consumption; a manufacturing of a durabledisplay device; an addition of heat resistance to the display device; amanufacturing of a more lightweight display device; or a manufacturingof a thinner display device. Examples of a substrate to which atransistor is transferred include, in addition to the substrate overwhich the transistor can be formed, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), and the like),a leather substrate, a rubber substrate, and the like.

<<Transistors 201 and 206>>

A transistor included in the display device of one embodiment of thepresent invention may have a top-gate structure or a bottom-gatestructure. Gate electrodes may be provided above and below a channel. Asemiconductor material used in the transistor is not particularlylimited, and an oxide semiconductor, silicon, or germanium can be used,for example.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistors, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle-crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. The use of a semiconductor havingcrystallinity is preferable as the degradation of a transistor'scharacteristics can be reduced.

For example, a Group 14 element, a compound semiconductor, or an oxidesemiconductor can be used for the semiconductor layer. Typically, asemiconductor including silicon, a semiconductor including galliumarsenide, or an oxide semiconductor including indium can be used for thesemiconductor layer.

An oxide semiconductor is preferably used for the semiconductor in whichthe channel of a transistor is formed. In particular, using an oxidesemiconductor with a larger bandgap than that of silicon is preferable.The use of a semiconductor material with a larger bandgap than that ofsilicon and a small carrier density is preferable because the currentduring the off state (off-state current) of the transistor can bereduced.

The oxide semiconductor preferably contains at least indium (In) or zinc(Zn). The oxide semiconductor further preferably contains an In-M-Znoxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn, orHf).

As the semiconductor layer, it is particularly preferable to use anoxide semiconductor layer including a plurality of crystal parts whosec-axes are aligned substantially perpendicular to a surface on which thesemiconductor layer is formed or the top surface of the semiconductorlayer and in which adjacent crystal parts have no grain boundary.

The use of such an oxide semiconductor for the semiconductor layer makesit possible to provide a highly reliable transistor in which a change inthe electrical characteristics is reduced.

Charge accumulated in a capacitor through the transistor can be retainedfor a long time because of low off-state current of the transistor. Theuse of such a transistor in pixels allows a driver circuit to stop whilethe gray level of a displayed image is maintained. As a result, adisplay device with extremely low power consumption is obtained.

The transistors 201 and 206 preferably include an oxide semiconductorlayer that is highly purified to reduce the formation of oxygenvacancies. This makes the off-state current of the transistor low.Accordingly, an electrical signal such as an image signal can be heldfor a long period, and a writing interval can be set long in an onstate. Thus, the frequency of refresh operation can be reduced, whichleads to an effect of reducing power consumption.

In the transistors 201 and 206, a relatively high field-effect mobilitycan be obtained, whereby high-speed operation is possible. The use ofsuch transistors that are capable of high-speed operation in the displaydevice enables the fabrication of the transistor in the display portionand the transistors in the driver circuit portion over the samesubstrate. This means that a semiconductor device separately formed witha silicon wafer or the like does not need to be used as the drivercircuit, which enables a reduction in the number of components in thedisplay device. In addition, using the transistor that can operate athigh speed in the display portion also can enable the provision of ahigh-quality image.

<<Oxide Semiconductor Layer>>

The oxide semiconductor layer preferably includes a film represented byan In-M-Zn oxide that contains at least indium (In), zinc (Zn), and M (ametal such as Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn, or Hf). In order toreduce variations in electrical characteristics of the transistorincluding the oxide semiconductor, the oxide semiconductor preferablycontains a stabilizer in addition to the In-M-Zn oxide.

Examples of the stabilizer, including metals that can be used as M, aregallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr).As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be used.

As an oxide semiconductor included in an oxide semiconductor layer, anyof the following can be used, for example: an In—Ga-based oxide, anIn—Zn-based oxide, an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, anIn—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide,an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-basedoxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, anIn—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide,an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-basedoxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, and an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main components and there is nolimitation on the ratio of In:Ga:Zn. Further, a metal element inaddition to In, Ga, and Zn may be contained.

Note that in the case where the oxide semiconductor layer includes anIn-M-Zn oxide, when the summation of In and M is assumed to be 100atomic %, the atomic proportions of In and M are preferably higher than25 atomic % and lower than 75 atomic %, respectively, more preferablyhigher than 34 atomic % and lower than 66 atomic %, respectively.

The energy gap of the oxide semiconductor layer is 2 eV or more,preferably 2.5 eV or more, more preferably 3 eV or more. The use of suchan oxide semiconductor having a wide energy gap leads to a reduction inoff-state current of a transistor.

The thickness of the oxide semiconductor layer is greater than or equalto 3 nm and less than or equal to 200 nm, preferably greater than orequal to 3 nm and less than or equal to 100 nm, and further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor layer includes an In-M-Znoxide (M is Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn, or Hf), it ispreferable that the atomic ratio of metal elements of a sputteringtarget used for forming a film of the In-M-Zn oxide satisfy In M and ZnM. As the atomic ratio of the metal elements of such a sputteringtarget, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=1:3:4,In:M:Zn=1:3:6, and the like are given. Note that the atomic ratio ofmetal elements in the formed oxide semiconductor layer varies from theabove atomic ratio of metal elements of the sputtering target within arange of ±40% as an error.

An oxide semiconductor layer with a low carrier density is used as theoxide semiconductor layer. For example, an oxide semiconductor layerwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, more preferably lower than or equalto 1×10¹³/cm³, and still more preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor layer.

Note that, without limitation to those described above, a material withan appropriate composition can be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of the transistor.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor layer, oxygen vacancies areincreased in the oxide semiconductor layer, and the oxide semiconductorlayer becomes an n-type. Thus, the concentration of silicon or carbon(the concentration is measured by SIMS) in the oxide semiconductor layeris lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than orequal to 2×10¹⁷ atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal inthe oxide semiconductor layer, which is measured by SIMS, is lower thanor equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶atoms/cm³. Alkali metal and alkaline earth metal can potentiallygenerate carriers when bonded to an oxide semiconductor, in which casethe off-state current of the transistor can potentially be increased.Therefore, it is preferable to reduce the concentration of alkali metalor alkaline earth metal in the oxide semiconductor layer.

When nitrogen is contained in the oxide semiconductor layer, electronsserving as carriers are generated and the carrier density increases, sothat the oxide semiconductor layer easily becomes n-type. Thus, atransistor including an oxide semiconductor that contains nitrogen islikely to be normally-on. For this reason, nitrogen in the oxidesemiconductor layer is preferably reduced as much as possible; theconcentration of nitrogen which is measured by SIMS is preferably setto, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor layer may have a non-single-crystal structure,for example. The non-single-crystal structure includes a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline structure,a microcrystalline structure, or an amorphous structure, for example.Among the non-single-crystal structure, the amorphous structure has thehighest density of defect states, whereas CAAC-OS has the lowest densityof defect states.

The oxide semiconductor layer may have an amorphous structure, forexample. An oxide semiconductor layer which has an amorphous structurehas a disordered atomic arrangement and no crystalline component, forexample. Alternatively, the oxide films having an amorphous structurehave, for example, an absolutely amorphous structure and no crystalpart.

Note that the oxide semiconductor layer may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a region of CAAC-OS, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Alternatively, the mixed filmmay have a layered structure of two or more of a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure.

<<Insulating Layer>>

An organic insulating material or an inorganic insulating material canbe used as an insulating material that can be used for the insulatinglayer, the overcoat, the spacer, or the like included in the displaydevice. Examples of an organic insulating material include an acrylicresin, an epoxy resin, a polyimide resin, a polyamide resin, apolyamide-imide resin, a siloxane resin, a benzocyclobutene-based resin,and a phenol resin. Examples of an inorganic insulating film include asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, and a neodymium oxide film

<<Conductive Layer>>

For the conductive layer such as the gate, the source, and the drain ofa transistor and the wiring, the electrode, and the like of the displaydevice, a single-layer structure or a layered structure using any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten, or an alloycontaining any of these metals as its main component can be used. Forexample, a two-layer structure in which a titanium film is stacked overan aluminum film; a two-layer structure in which a titanium film isstacked over a tungsten film; a two-layer structure in which a copperfilm is stacked over a molybdenum film; a two-layer structure in which acopper film is stacked over an alloy film containing molybdenum andtungsten; a two-layer structure in which a copper film is stacked overan alloy film containing copper, magnesium, and aluminum; a three-layerstructure in which titanium film or a titanium nitride film, an aluminumfilm or a copper film, and a titanium film or a titanium nitride filmare stacked in this order; a three-layer structure in which a molybdenumfilm or a molybdenum nitride film, an aluminum film or a copper film,and a molybdenum film or a molybdenum nitride film are stacked in thisorder; or the like can be employed. For example, in the case where theconductive layer has a three-layer structure, it is preferable that eachof the first and third layers be a film formed of titanium, titaniumnitride, molybdenum, tungsten, an alloy containing molybdenum andtungsten, an alloy containing molybdenum and zirconium, or molybdenumnitride, and that the second layer be a film formed of a low-resistancematerial such as copper, aluminum, gold, silver, or an alloy containingcopper and manganese. Note that light-transmitting conductive materialssuch as ITO, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or ITSOmay be used.

An oxide conductive layer may be formed by controlling the resistivityof the oxide semiconductor.

<<Adhesive Layer 141>>

A curable resin such as a heat-curable resin, a photocurable resin, or atwo-component type curable resin can be used for the adhesive layer 141.For example, an acrylic resin, a urethane resin, an epoxy resin, or asiloxane resin can be used.

<<Connector 242>>

As the connectors 242 and 243, for example, an anisotropic conductivefilm (ACF) or an anisotropic conductive paste (ACP) can be used.

<<Coloring Layer 131>>

The coloring layer 131 is a colored layer that transmits light in aspecific wavelength range. Examples of materials that can be used forthe coloring layer 131 include a metal material, a resin material, and aresin material containing a pigment or dye.

<<Light-Blocking Layer 132>>

The light-blocking layer 132 is provided, for example, between adjacentcoloring layers 131 for different colors. A black matrix formed with,for example, a metal material or a resin material containing a pigmentor dye can be used as the light-blocking layer 132. Note that it ispreferable to provide the light-blocking layer 132 also in a regionother than the display portion 62, such as the driver circuit portion64, in which case undesired leakage of guided light or the like can beinhibited.

The thin films constituting the display device (i.e., the insulatingfilm, the semiconductor film, the conductive film, and the like) can beformed by any of a sputtering method, a chemical vapor deposition (CVD)method, a vacuum evaporation method, a pulsed laser deposition (PLD)method, an atomic layer deposition (ALD) method, and the like. Asexamples of the CVD method, a plasma-enhanced CVD (PECVD) method or athermal CVD method can be given. As an example of the thermal CVDmethod, a metal organic CVD (MOCVD) method can be given.

Alternatively, the thin films constituting the display device (i.e., theinsulating film, the semiconductor film, the conductive film, and thelike) can be formed by a method such as spin coating, dipping, spraycoating, inkjet printing, dispensing, screen printing, or offsetprinting, or with a doctor knife, a slit coater, a roll coater, acurtain coater, or a knife coater.

The thin films constituting the display device can be processed using aphotolithography method or the like. Alternatively, island-shaped thinfilms may be formed by a film formation method using a blocking mask.Alternatively, the thin films may be processed by a nano-imprintingmethod, a sandblasting method, a lift-off method, or the like. Examplesof the photolithography method include a method in which a resist maskis formed over a thin film to be processed, the thin film is processedby etching or the like, and the resist mask is removed, and a method inwhich a photosensitive thin film is formed, and the photosensitive thinfilm is exposed to light and developed to be processed in a desiredshape.

As light used in exposure in a photolithography method, light with ani-line (with a wavelength of 365 nm), light with a g-line (with awavelength of 436 nm), light with an h-line (with a wavelength of 405nm), and light in which the i-line, the g-line, and the h-line are mixedcan be given. Alternatively, ultraviolet light, KrF laser light, ArFlaser light, or the like can be used. Exposure may be performed byliquid immersion exposure technique. As light used in exposure, extremeultra-violet light (EUV), X-rays, or the like can be given. An electronbeam can be used instead of a light used in exposure. It is preferableto use extreme ultra-violet light, X-rays, or an electron beam becauseextremely minute processing can be performed. Note that when exposure isperformed by scanning of a beam such as an electron beam, a photomask isnot needed.

For etching of the thin film, dry etching, wet etching, a sandblastmethod, or the like can be used.

<1-3. Structure Example 3 of Display Device>

FIG. 7 to FIG. 10 illustrate examples of the display device. FIG. 7 is across-sectional view of a display device 100C, FIG. 8A is across-sectional view of a display device 100D, FIG. 9A is across-sectional view of a display device 100E, and FIG. 10 is across-sectional view of a display device 100F. Note that the perspectiveviews of the display devices 100C, 100D, 100E, and 100F are not drawnhere, as they are similar to the perspective view of the display device100A, which is illustrated in FIG. 3A.

The display device 100C illustrated in FIG. 7 is different from theabove-described display device 100A in the positions of the pixelelectrode 111 and the first common electrode 112.

In the display device 100A illustrated in FIG. 4A, the alignment film133 a is in contact with the first common electrode 112. In contrast, inthe display device 100C illustrated in FIG. 7 , the alignment film 133 ais in contact with the pixel electrode 111.

The display device 100D illustrated in FIGS. 8A to 8D is different fromthe display device 100A in the shapes of the pixel electrode 111 and thefirst common electrode 112.

Both of the pixel electrode 111 and the first common electrode 112 mayhave a top-surface shape (also referred to as a planar shape) that has acomb-like shape or a top-surface shape that is provided with a slit.

In the display device 100D illustrated in FIGS. 8A to 8D, the pixelelectrode 111 and the first common electrode 112 are provided on thesame plane.

Alternatively, the electrodes may have a shape in which an edge of aslit in one electrode is aligned with an edge of a slit in the otherelectrode. The cross-sectional view of this case is shown in FIG. 8B.

Alternatively, the pixel electrode 111 and the first common electrode112 may have a portion overlapping with each other, when seen fromabove. The cross-sectional view of this case is shown in FIG. 8C.

Alternatively, the display portion 62 may have a portion where neitherthe pixel electrode 111 nor the first common electrode 112 is provided,when seen from above. The cross-sectional view of this case is shown inFIG. 8D.

The display device 100E illustrated in FIG. 9A and the display device100F illustrated in FIG. 10 each are different from the display device100A in the shapes of the transistors.

In FIG. 9A, each of the transistors 201 and 206 includes the gate 221,the insulating layer 213, the conductive layers 222 a and 222 b, and asemiconductor layer 231.

The gate 221 and the semiconductor layer 231 overlap with the insulatinglayer 213 positioned therebetween. The insulating layer 213 serves as agate insulating layer. Each of the conductive layers 222 a and 222 b hasa portion connected to the semiconductor layer 231. One of theconductive layers 222 a and 222 b serves as a source electrode and theother serves as a drain electrode. The transistors 201 and 206 arecovered by the insulating layers 212 and 214.

In FIG. 9A, the pixel electrode 111 is connected to the conductive layer222 b. Alternatively, the pixel electrode 111 may be connected to thesemiconductor layer 231 as illustrated in FIG. 9B. In that case, amaterial transmitting visible light, such as an oxide semiconductor, ispreferably used for the semiconductor layer 231. This allows the pixelelectrode 111 and the connection portion of the transistor to beprovided in the display region 68, increasing the aperture ratio of thesubpixel and the definition of the display device. Note that thesemiconductor layer 231 may be electrically connected to the conductivelayer 222 b. The conductive layer 222 b can serve as an auxiliaryelectrode of the semiconductor layer 231. The transistor does notnecessarily include the conductive layer 222 b.

In FIG. 10 , each of the transistors 201 and 206 includes the gate 221,the gate 223, the insulating layers 212 to 214, the conductive layers222 a and 222 b, and the semiconductor layer 231.

The gate 221 and the semiconductor layer 231 overlap with the insulatinglayer 213 positioned therebetween. The gate 223 and the semiconductorlayer 231 overlap with the insulating layers 212 and 214 positionedtherebetween. Each of the insulating layers 212 to 214 serves as a gateinsulating layer. Each of the conductive layers 222 a and 222 b has aportion connected to the semiconductor layer 231. One of the conductivelayers 222 a and 222 b serves as a source electrode and the other servesas a drain electrode. The transistors 201 and 206 are covered by theinsulating layer 215. The conductive layer 222 b is connected to thepixel electrode 111.

As described above, the display device of one embodiment of the presentinvention can include transistors and liquid crystal elements withvarious shapes.

<1-4. Structure Example 4 of Display Device>

One embodiment of the present invention can be applied to a displaydevice in which a touch sensor is implemented; such a display device isalso referred to as an input/output device or a touch panel. Any of thestructures of the display device described above can be applied to thetouch panel. In this embodiment, the description focuses on an examplein which the touch sensor is implemented in the display device 100A.

There is no limitation on the sensing element (also referred to as asensor element) included in the touch panel of one embodiment of thepresent invention. A variety of sensors capable of sensing an approachor a contact of an object such as a finger or a stylus can be used asthe sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, a touch panel including a capacitive sensor elementis described as an example.

Examples of the capacitive touch sensor element include a surfacecapacitive touch sensor element and a projected capacitive touch sensorelement. Examples of the projected capacitive sensor element include aself-capacitive sensor element and a mutual capacitive sensor element.The use of a mutual capacitive sensor element is preferable becausemultiple points can be sensed simultaneously.

The touch panel of one embodiment of the present invention can have anyof a variety of structures, including a structure in which a displaydevice and a sensor element that are separately formed are attached toeach other and a structure in which an electrode and the like includedin a sensor element are provided on one or both of a substratesupporting a display element and a counter substrate.

FIGS. 11A and 11B and FIG. 12 illustrate an example of the touch panel.FIG. 11A is a perspective view of a touch panel 350A. FIG. 11B is adeveloped view of the schematic perspective view of FIG. 11A. Note thatfor simplicity, FIGS. 11A and 11B illustrate only the major components.In FIG. 11B, the outlines of the substrate 61 and a substrate 162 areillustrated only in dashed lines. FIG. 12 is a cross-sectional view ofthe touch panel 350A.

The touch panel 350A has a structure in which a display device and asensor element that are fabricated separately are bonded together.

The touch panel 350A includes an input device 375 and a display device370 that are provided to overlap with each other.

The input device 375 includes the substrate 162, an electrode 127, anelectrode 128, a plurality of wirings 137, and a plurality of wirings138. An FPC 72 b is electrically connected to each of the plurality ofwirings 137 and the plurality of wirings 138. An IC 73 b is provided onthe FPC 72 b.

The display device 370 includes the substrate 51 and the substrate 61which are provided to face each other. The display device 370 includesthe display portion 62 and the driver circuit portion 64. The wiring 65and the like are provided over the substrate 51. An FPC 72 a iselectrically connected to the wiring 65. An IC 73 a is provided on theFPC 72 a.

The wiring 65 supplies signals and power to the display portion 62 andthe driver circuit portion 64. The signals and power are input to thewiring 65 from the outside or the IC 73 a, through the FPC 72 a.

FIG. 12 is a cross-sectional view of the display portion 62, the drivercircuit portion 64, a region that includes the FPC 72 a, a region thatincludes the FPC 72 b, and the like.

The substrates 51 and 61 are bonded to each other by the adhesive layer141. The substrates 61 and 162 are bonded to each other by an adhesivelayer 169. Here, the layers from the substrate 51 to the substrate 61correspond to the display device 370. The layers from the substrate 162to an electrode 124 correspond to the input device 375. That is, theadhesive layer 169 bonds the display device 370 and the input device 375together.

The structure of the display device 370 illustrated in FIG. 12 issimilar to that of the display device 100A illustrated in FIG. 4A;detailed description is omitted here.

A polarizer 165 is bonded to the substrate 51 with an adhesive layer167. A backlight 161 is bonded to the polarizer 165 with an adhesivelayer 163.

A polarizer 166 is bonded to the substrate 162 with an adhesive layer168. A protection substrate 160 is bonded to the polarizer 166 with anadhesive layer 164. The protection substrate 160 may be used as thesubstrate that objects such as a finger or a stylus directly contact,when the touch panel 350A is incorporated into an electronic device. Asubstrate that can be used as the substrates 51 and 61 or the like canbe used as the protection substrate 160. A structure where a protectivelayer is formed on the surface of the substrate that can be used as thesubstrates 51 and 61 or the like is preferably used for the protectionsubstrate 160. Alternatively, a reinforced glass or the like ispreferably used as the protection substrate 160. The protective layercan be formed with a ceramic coating. The protective layer can be formedusing an inorganic insulating material such as silicon oxide, aluminumoxide, yttrium oxide, or yttria-stabilized zirconia (YSZ).

The polarizer 166 may be provided between the input device 375 and thedisplay device 370. In that case, the protection substrate 160, theadhesive layer 164, and the adhesive layer 168 that are illustrated inFIG. 12 are not necessarily provided. In other words, the substrate 162can be positioned on the outermost surface of the touch panel 350A. Theabove-described material that can be used for the protection substrate160 is preferably used for the substrate 162.

The electrodes 127 and 128 are provided on a surface of the substrate162 that faces the substrate 61. The electrodes 127 and 128 are formedon the same plane. An insulating layer 125 is provided to cover theelectrodes 127 and 128. The electrode 124 is electrically connected totwo of the electrodes 128 that are provided on both sides of theelectrode 127, through an opening provided in the insulating layer 125.

In the conductive layers included in the input device 375, theconductive layers (e.g., the electrodes 127 and 128) that overlap withthe display region 68 are formed using a material that transmits visiblelight.

The wiring 137 that is obtained by processing the same conductive layeras the electrodes 127 and 128 is connected to a conductive layer 126that is obtained by processing the same conductive layer as theelectrode 124. The conductive layer 126 is electrically connected to theFPC 72 b through a connector 242 b.

Next, an example of a driving method of an input device (touch sensor)that can be applied to the display device of one embodiment of thepresent invention is described with reference to FIGS. 13A and 13B.

FIG. 13A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 13A illustrates a pulse voltage outputcircuit 601 and a current sensing circuit 602. In FIG. 13A, six wiringsX1 to X6 represent electrodes 621 to which a pulse is applied, and sixwirings Y1 to Y6 represent electrodes 622 that sense changes in current.The number of such electrodes is not limited to those illustrated inthis example. FIG. 13A also illustrates a capacitor 603 that is formedby the overlap of the electrodes 621 and 622, or by the closearrangement of the electrodes 621 and 622. Note that the functions ofthe electrodes 621 and 622 may change places with each other.

For example, the electrode 127 corresponds to one of the electrode 621or the electrode 622, and the electrode 128 corresponds to the other ofthe electrode 621 or the electrode 622.

The pulse voltage output circuit 601 is, for example, a circuit forsequentially inputting a pulse voltage to the wirings X1 to X6. Thecurrent sensing circuit 602 is, for example, a circuit for sensingcurrent flowing through each of the wirings Y1 to Y6.

An application of a pulse voltage to one of the wirings X1 to X6generates an electric field between the electrodes 621 and 622 of thecapacitor 603, and current flows through the electrode 622. Part of theelectric field generated between the electrodes is blocked when anobject such a finger or a stylus approaches or contacts the device, sothat the electric field intensity between the electrodes is changed.Consequently, the amount of current flowing through the electrode 622 ischanged.

For example, in the case where there is no approach or no contact of anobject, the amount of current flowing in each of the wirings Y1 to Y6depends on the capacitance of the capacitor 603. In the case where partof an electric field is blocked by the approach or contact of an object,a decrease in the amount of current flowing in the wirings Y1 to Y6 issensed. The approach or contact of an object can be detected byutilizing this change.

The current sensing circuit 602 may sense an integral value (timeintegral value) of current flowing in a wiring. In that case, forexample, an integrator circuit can be used. Alternatively, the peakvalue of current may be sensed. In that case, for example, current maybe converted into voltage, and the peak voltage value may be sensed.

FIG. 13B is an example of a timing chart illustrating input and outputwaveforms in the mutual capacitive touch sensor in FIG. 13A. In FIG.13B, sensing in each row and each column is performed in one sensingperiod. FIG. 13B shows a period when the approach or contact of anobject is not detected (when the touch sensor is not touched) and aperiod when the approach or contact of an object is detected (when thetouch sensor is touched). Here, the wirings Y1 to Y6 each show awaveform of a voltage corresponding to the amount of current to besensed.

As shown in FIG. 13B, the wirings X1 to X6 are sequentially suppliedwith a pulse voltage. Accordingly, current flows in the wirings Y1 toY6. When the touch sensor is not touched, substantially the same currentflows in the wirings Y1 to Y6 in accordance with a change in voltages ofthe wirings X1 to X6; thus, the wirings Y1 to Y6 have similar outputwaveforms. Meanwhile, when the touch sensor is touched, current flowingin a wiring in a position which an object contacts or approaches amongthe wirings Y1 to Y6 is reduced; thus, the output waveforms are changedas illustrated in FIG. 13B.

FIG. 13B shows an example in which an object contacts or approaches theintersection of the wiring X3 and the wiring Y3 or the vicinity thereof.

A mutual capacitive touch sensor senses a change in current which occursdue to an electric field generated between a pair of electrodes beingblocked; the mutual capacitive touch sensor can obtain positionalinformation of an object in this manner. When the sensing sensitivity ishigh, the coordinates of the object can be determined even when theobject is far from a detection surface (e.g., a surface of the touchpanel).

By driving a touch panel by a method in which a display period of adisplay portion and a sensing period of a touch sensor do not overlapwith each other, the detection sensitivity of the touch sensor can beincreased. For example, a display period and a sensing period may beseparately provided in one display frame period. In that case, two ormore sensing periods are preferably provided in one frame period. Whenthe sensing frequency is increased, the detection sensitivity can befurther increased.

It is preferable that, as an example, the pulse voltage output circuit601 and the current sensing circuit 602 be formed in an IC chip. Forexample, the IC is preferably mounted on a touch panel or a substrate ina housing of an electronic device. In the case where the touch panel hasflexibility, parasitic capacitance can potentially be increased in abent portion of the touch panel, and the influence of noise canpotentially be increased. In view of this, an IC with a driving methodless influenced by noise is preferably used. For example, it ispreferable to use an IC to which a driving method capable of increasinga signal-noise ratio (S/N ratio) is applied.

<1-5. Structure Example 5 of Display Device>

Examples of the touch panel are illustrated in FIGS. 14A to 14C and FIG.15 . FIG. 14A is a perspective view of a touch panel 350B. FIG. 14B is adeveloped view of the schematic perspective view of FIG. 14A. Note thatfor simplicity, FIGS. 14A and 14B illustrate only the major components.In FIG. 14B, the outlines of the substrate 61 are illustrated only indashed lines. FIG. 15 is a cross-sectional view of the touch panel 350B.

The touch panel 350B is an in-cell touch panel that has a function ofdisplaying an image and serves as a touch sensor.

The touch panel 350B has a structure in which electrodes constituting asensor element and the like are provided only on the counter substrate.Such a structure can make the touch panel thinner and more lightweightor reduce the number of components within the touch panel, compared witha structure in which the display device and the sensor element arefabricated separately and then are bonded together.

In FIGS. 14A and 14B, an input device 376 is provided on the substrate61. The wirings 137 and 138 and the like of the input device 376 areelectrically connected to the FPC 72 included in a display device 379.

With the above structure, the FPCs connected to the touch panel 350B canbe provided only on one substrate side (on the substrate 51 side in thisembodiment). Although two or more FPCs may be attached to the touchpanel 350B, it is preferable that the touch panel 350B be provided withone FPC 72 which has a function of supplying signals to both the displaydevice 379 and the input device 376 as illustrated in FIGS. 14A and 14B,for the simplicity of the structure.

The IC 73 may include a function of driving the input device 376.Another IC that drives the input device 376 may be provided over the FPC72. Alternatively, an IC that drives the input device 376 may be mountedon the substrate 51.

FIG. 15 is a cross-sectional view including a region that includes theFPC 72, a connection portion 63, the driver circuit portion 64, and thedisplay portion 62, each of which is illustrated in FIG. 14A.

In the connection portion 63, one of the wiring 137 (or the wiring 138)and one of the conductive layers provided on the substrate 51 side areelectrically connected through the connector 243.

The light-blocking layer 132 is provided in contact with the substrate61, thereby preventing the conductive layers used in the touch sensorfrom being seen by a user. The light-blocking layer 132 is covered by aninsulating layer 122. The electrode 127 is provided between theinsulating layer 122 and the insulating layer 125. The electrode 128 isprovided between the insulating layer 125 and the insulating layer 123.The electrodes 127 and 128 can be formed using a metal or an alloy. Thecoloring layer 131 is provided in contact with the insulating layer 123.Note that as illustrated in FIG. 16 , a light-blocking layer 132 a maybe provided in contact with the insulating layer 123 in addition to alight-blocking layer 132 b that is in contact with the substrate 61.

The wiring 137 that is obtained by processing the same conductive layeras the electrode 127 is connected to a conductive layer 285 that isobtained by processing the same conductive layer as the electrode 128.The conductive layer 285 is connected to the conductive layer 286 thatis obtained by processing the same conductive layer as the second commonelectrode 244. The conductive layer 286 is electrically connected to theconductive layer 284 through the connector 243.

The touch panel 350B is supplied with a signal for driving a pixel and asignal for driving a sensor element from one FPC. Thus, the touch panel350B can easily be incorporated into an electronic device and allows areduction in the number of components.

<1-6. Structure Example of Touch Sensor>

A structure example of the input device (touch sensor) will be describedbelow.

FIG. 17A is a top view of an input device 415. The input device 415includes a plurality of electrodes 471, a plurality of electrodes 472, aplurality of wirings 476, and a plurality of wirings 477 over asubstrate 416. The substrate 416 is provided with an FPC 450 that iselectrically connected to each of a plurality of wirings 476 and aplurality of wirings 477. FIG. 17A illustrates an example in which an IC449 is provided on the FPC 450.

FIG. 17B is an enlarged view of a region surrounded by a dashed-dottedline in FIG. 17A. The electrodes 471 are in the form of a row of rhombicelectrode patterns arranged in a lateral direction. The row of rhombicelectrode patterns are electrically connected to each other. Theelectrodes 472 are also in the form of a row of rhombic electrodepatterns arranged in a longitudinal direction, and the row of rhombicelectrode patterns are electrically connected. Part of the electrodes471 and part of the electrodes 472 overlap and intersect with eachother. At this intersection portion, an insulator is sandwiched betweenthe electrodes 471 and the electrodes 472 in order to avoid anelectrical short-circuit therebetween.

As illustrated in FIG. 17C, the electrodes 472 may include a pluralityof island-shaped rhombic electrodes 473 and bridge electrodes 474. Theisland-shaped rhombic electrodes 473 are arranged in the longitudinaldirection, and two adjacent electrodes 473 are electrically connected toeach other by the bridge electrode 474. With such a structure, theelectrodes 473 and the electrodes 471 can be formed at the same time byprocessing the same conductive film. This can prevent variations in thethickness of these electrodes, and can prevent the resistance value andthe light transmittance of each electrode from varying from place toplace. Note that although the electrodes 472 include the bridgeelectrodes 474 here, the electrodes 471 may have such a structure.

As illustrated in FIG. 17D, a design in which rhombic electrode patternsof the electrodes 471 and 472 illustrated in FIG. 17B are hollowed outand only edge portions are left may be used. In that case, when theelectrodes 471 and 472 are narrow enough to be invisible to the users,the electrodes 471 and 472 can be formed using a light-blocking materialsuch as a metal or an alloy, as will be described later. In addition,either the electrodes 471 or the electrodes 472 illustrated in FIG. 17Dmay include the above bridge electrodes 474.

One of the electrodes 471 is electrically connected to one of thewirings 476. One of the electrodes 472 is electrically connected to oneof the wirings 477. Here, either one of the electrodes 471 and 472corresponds to a row wiring, and the other corresponds to a columnwiring.

The IC 449 has a function of driving the touch sensor. A signal outputfrom the IC 449 is supplied to either of the electrodes 471 and 472through the wirings 476 or 477. A current (or a potential) flowing toeither of the electrodes 471 and 472 is input to the IC 449 through thewirings 476 or 477. The IC 449 is mounted on the FPC 450 in thisexample; alternatively, the IC 449 may be mounted on the substrate 416.

When the input device 415 overlaps with a display screen of the displaypanel, a light-transmitting conductive material is preferably used forthe electrodes 471 and 472. In the case where a light-transmittingconductive material is used for the electrodes 471 and 472 and lightfrom the display panel is extracted through the electrodes 471 or 472,it is preferable that a conductive film containing the same conductivematerial be arranged between the electrodes 471 and 472 as a dummypattern. Part of a space between the electrodes 471 and 472 is filledwith the dummy pattern, which can reduce variations in lighttransmittance. As a result, unevenness in luminance of light transmittedthrough the input device 415 can be reduced.

As the light-transmitting conductive material, a conductive oxide suchas indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, orzinc oxide containing gallium can be used. Note that a film containinggraphene may be used as well. The film containing graphene can beformed, for example, by reducing a film containing graphene oxide. As areducing method, a method with application of heat or the like can beemployed.

Alternatively, a metal film or an alloy film that is thin enough to havea light-transmitting property can be used. For example, a metal such asgold, silver, platinum, magnesium, nickel, tungsten, chromium,molybdenum, iron, cobalt, copper, palladium, or titanium, or an alloycontaining any of these metals can be used. Alternatively, a nitride ofthe metal or the alloy (e.g., titanium nitride) or the like may be used.Alternatively, a layered film in which two or more of conductive filmscontaining the above materials are stacked may be used.

For the electrodes 471 and 472, a conductive film that is processed tobe thin enough to be invisible to the users may be used. Such aconductive film is processed into a lattice shape (a mesh shape), forexample, which makes it possible to achieve both high conductivity andhigh visibility of the display device. It is preferable that theconductive film have a portion with a width greater than or equal to 30nm and less than or equal to 100 μm, preferably greater than or equal to50 nm and less than or equal to 50 μm, and further preferably greaterthan or equal to 50 nm and less than or equal to 20 μm. In particular,the conductive film preferably has a pattern width of 10 μm or less,which is hardly visible to the users.

As examples, enlarged schematic views of a region 460 in FIG. 17B areillustrated in FIGS. 18A to 18D.

FIG. 18A illustrates an example where a lattice-shape conductive film461 is used. The conductive film 461 is preferably placed so as not tooverlap with the display element included in the display device becauselight from the display element is not blocked. In that case, it ispreferable that the direction of the lattice be the same as thedirection of the display element arrangement and that the pitch of thelattice be an integer multiple of the pitch of the display elementarrangement.

FIG. 18B illustrates an example of a lattice-shape conductive film 462,which is processed so as to be provided with triangle openings. Such astructure makes it possible to further reduce the resistance comparedwith the structure illustrated in FIG. 18A.

Alternatively, a conductive film 463, which has an irregular patternshape, may be used as illustrated in FIG. 18C. Such a structure canprevent generation of moire when overlapping with the display portion ofthe display device.

Conductive nanowires may be used for the electrodes 471 and 472. FIG.18D illustrates an example of using nanowires 464. The nanowires 464 aredispersed at appropriate density so as to be in contact with theadjacent nanowires, which can form a two-dimensional network; thenanowires 464 can function as a conductive film with extremely highlight-transmitting property. For example, nanowires that have a meandiameter of greater than or equal to 1 nm and less than or equal to 100nm, preferably greater than or equal to 5 nm and less than or equal to50 nm, and further preferably greater than or equal to 5 nm and lessthan or equal to 25 nm, can be used. As the nanowire 464, a metalnanowire such as an Ag nanowire, a Cu nanowire, or an Al nanowire, acarbon nanotube, or the like can be used. In the case of using an Agnanowire, a light transmittance of 89% or more and a sheet resistance of40 ohms per square or more and 100 ohms per square or less can beachieved.

FIG. 18E illustrates a more specific structure example of the electrodes471 and 472 in FIG. 17B. FIG. 18E shows an example in which alattice-shape conductive film is used for each of the electrodes 471 and472.

Although examples in which a plurality of rhombuses are aligned in onedirection are shown in FIG. 17A and the like as top surface shapes ofthe electrodes 471 and 472, the shapes of the electrodes 471 and 472 arenot limited thereto and can have various top surface shapes such as abelt shape (a rectangular shape), a belt shape having a curve, and azigzag shape. In addition, although the above shows the electrodes 471and 472 are arranged to be perpendicular to each other, they are notnecessarily arranged to be perpendicular and the angle formed by two ofthe electrodes may be less than 90°.

<1-7. Structure Example 6 of Display Device>

An example of the touch panel is illustrated in FIG. 19 . FIG. 19 is across-sectional view of a touch panel 350D.

The touch panel 350D is an in-cell touch panel that has a function ofdisplaying an image and serves as a touch sensor.

The touch panel 350D has a structure in which electrodes constituting asensor element and the like are provided only on a substrate thatsupports a display element. Such a structure can make the touch panelthinner and more lightweight or reduce the number of components withinthe touch panel, compared with a structure in which the display deviceand the sensor element are fabricated separately and then are bondedtogether or a structure in which the sensor element is fabricated on thecounter substrate side.

The touch panel 350D illustrated in FIG. 19 is different from thedisplay device 100A described above in the layout of the commonelectrode and the auxiliary wiring 139.

A plurality of auxiliary wirings 139 are electrically connected to thefirst common electrode 112 a or the first common electrode 112 b.

The touch panel 350D illustrated in FIG. 19 is capable of sensing anapproach or a contact or the like of an object utilizing the capacitanceformed between the first common electrode 112 a and the first commonelectrode 112 b. That is, in the touch panel 350D, the first commonelectrodes 112 a and 112 b serve as both the common electrode of theliquid crystal element and the electrode of the sensor element.

As described above, an electrode of the liquid crystal element alsoserves as an electrode of the sensor element in the touch panel of oneembodiment of the present invention; thus, the manufacturing process canbe simplified and the manufacturing cost can be reduced. Furthermore,the touch panel can be made thin and lightweight.

The common electrode is electrically connected to the auxiliary wiring139. By providing the auxiliary wiring 139, the resistance of theelectrodes of the sensor element can be reduced. As the resistance ofthe electrodes of the sensor element is reduced, the time constant ofthe electrode of the sensor element can be made small. When the timeconstant of the electrode of the sensor element is smaller, thedetection sensitivity can be increased, which enables an increase indetection accuracy.

For example, the time constant of the electrode of the sensor element isgreater than 0 seconds and less than or equal to 1×10 seconds,preferably greater than 0 seconds and less than or equal to 5×10⁻⁵seconds, further preferably greater than 0 seconds and less than orequal to 5×10⁻⁶ seconds, further preferably greater than 0 seconds andless than or equal to 5×10⁻⁷ seconds, and further preferably greaterthan 0 seconds and less than or equal to 2×10⁻⁷ seconds. In particular,when the time constant is smaller than or equal to 1×10⁻⁶ seconds, highdetection sensitivity can be achieved while the influence of noise isreduced.

The signal for driving a pixel and the signal for driving a sensorelement are supplied to the touch panel 350D by one FPC. Thus, the touchpanel 350D can easily be incorporated into an electronic device andallows a reduction in the number of components.

An example of the operation method of the touch panel 350D and the likewill be described below.

FIG. 20A is an equivalent circuit diagram of part of a pixel circuitprovided in the display portion 62 of the touch panel 350D.

Each pixel (subpixel) includes at least the transistor 206 and theliquid crystal element 40. The gate of the transistor 206 iselectrically connected to a wiring 3501. One of the source and the drainof the transistor 206 is electrically connected to a wiring 3502.

The pixel circuit includes a plurality of wirings extending in the Xdirection (e.g., a wiring 3510_1 and a wiring 3510_2) and a plurality ofwirings extending in the Y direction (e.g., a wiring 3511_1). They areprovided to intersect with each other, and capacitance is formedtherebetween.

Among the pixels provided in the pixel circuit, electrodes of the liquidcrystal elements of some pixels adjacent to each other are electricallyconnected to each other to form one block. The block is classified intotwo types: an island-shaped block (e.g., a block 3515_1 or a block35152), and a linear block extending in the X direction or the Ydirection (e.g., a block 3516 extending in the Y direction). Note thatonly part of the pixel circuit is illustrated in FIG. 20A, and inreality, these two types of blocks are repeatedly arranged in the Xdirection and the Y direction. An electrode on one side of the liquidcrystal element is, for example, a common electrode. An electrode on theother side of the liquid crystal element is, for example, a pixelelectrode.

The wiring 3510_1 (or the wiring 35102) extending in the X direction iselectrically connected to the island-shaped block 3515_1 (or the block35152). Although not illustrated, the wiring 3510_1 extending in the Xdirection is electrically connected to a plurality of island-shapedblocks 3515_1 which are provided discontinuously along the X directionwith the linear blocks therebetween. Furthermore, the wiring 3511_1extending in the Y direction is electrically connected to the linearblock 3516.

FIG. 20B is an equivalent circuit diagram illustrating the connectionrelation between a plurality of wirings extending in the X direction(the wirings 3510_1 to 3510_6, which are collectively called a wiring3510 in some cases) and a plurality of wirings extending in the Ydirection (wirings 3511_1 to 3511_6, which are collectively called awiring 3511 in some cases). A common potential can be input to each ofthe wirings 3510 extending in the X direction and each of the wirings3511 extending in the Y direction. A pulse voltage can be input to eachof the wirings 3510 extending in the X direction from a pulse voltageoutput circuit. Furthermore, each of the wirings 3511 extending in the Ydirection can be electrically connected to the sensing circuit. Notethat the wiring 3510 and the wiring 3511 can be interchanged with eachother.

An example of an operation method of the touch panel 350D is describedwith reference to FIGS. 21A and 21B.

Here, one frame period is divided into a writing period and a sensingperiod. The writing period is a period in which image data is written toa pixel, and the wirings 3501 (also referred to as gate lines or scanlines) are sequentially selected. The sensing period is a period inwhich sensing is performed by the sensor element.

FIG. 21A is an equivalent circuit diagram in the writing period. In thewriting period, a common potential is input to both the wiring 3510extending in the X direction and the wiring 3511 extending in the Ydirection.

FIG. 21B is an equivalent circuit diagram in the sensing period. In thesensing period, each of the wirings 3511 extending in the Y direction iselectrically connected to the detection circuit. Furthermore, a pulsevoltage is input to the wirings 3510 extending in the X direction from apulse voltage output circuit.

FIG. 21C illustrates an example of a timing chart of the input andoutput waveforms of a mutual capacitive sensor element.

In FIG. 21C, sensing of an object is performed in all rows and columnsin one frame period. FIG. 21C shows two cases in the sensing period: acase in which an object is not sensed (not touched) and a case in whichan object is sensed (touched).

A pulse voltage is supplied to the wirings 3510_1 to 3510_6 from thepulse voltage output circuit. When the pulse voltage is applied to thewirings 3510_1 to 3510_6, an electric field is generated between a pairof electrodes forming a capacitor, and current flows in the capacitor.The electric field generated between the electrodes is changed by beingblocked by the touch of a finger or a stylus, for example That is, thecapacitance value of the capacitor is changed by touch or the like. Byutilizing this, an approach or contact of an object can be sensed.

The wirings 3511_1 to 3511_6 are connected to the detection circuit fordetecting the change in current in the wirings 3511_1 to 3511_6 causedby the change in capacitance value of the capacitor. The current valuedetected in the wirings 3511_1 to 3511_6 is not changed when there is noapproach or contact of an object, and is decreased when the capacitancevalue is decreased because of the approach or contact of an object. Inorder to detect a change in current, the total amount of current may bedetected. In that case, an integrator circuit or the like may be used todetect the total amount of current. Alternatively, the peak currentvalue may be detected. In that case, current may be converted intovoltage, and the peak voltage value may be detected.

Note that in FIG. 21C, the waveforms of the wirings 3511_1 to 3511_6show voltage values corresponding to the detected current values. Asillustrated in FIG. 21C, the timing of the display operation ispreferably in synchronization with the timing of the sensing operation.

The waveforms of the wirings 3511_1 to 3511_6 change in accordance withpulse voltages applied to the wirings 3510_1 to 3510_6. When there is noapproach or contact of an object, the waveforms of the wirings 3511_1 to3511_6 uniformly change in accordance with changes in the voltages ofthe wirings 3510_1 to 3510_6. On the other hand, the current valuedecreases at the point of approach or contact of an object andaccordingly the waveform of the voltage value changes.

By detecting a change in capacitance in this manner, the approach orcontact of an object can be detected. Even when an object such as afinger or a stylus does not touch but only approaches a touch panel, asignal may be detected in some cases.

Note that FIG. 21C illustrates an example in which a common potentialsupplied in the writing period is equal to a low potential supplied inthe sensing period in the wiring 3510; however, one embodiment of thepresent invention is not limited thereto. The common potential may bedifferent from the low potential.

It is preferable that, as an example, the pulse voltage output circuitand the detection circuit be formed in one IC. For example, the IC ispreferably mounted on a touch panel or a substrate in a housing of anelectronic device. In the case where the touch panel has flexibility,parasitic capacitance can potentially be increased in a bent portion ofthe touch panel, and the influence of noise can potentially beincreased. In view of this, an IC with a driving method less influencedby noise is preferably used. For example, it is preferable to use an ICto which a driving method capable of increasing a signal-noise ratio(S/N ratio) is applied.

It is preferable that a period in which an image is written and a periodin which sensing is performed by a sensor element be separately providedas described above. Thus, a decrease in sensitivity of the sensorelement caused by noise generated when data is written to a pixel can beprevented.

In one embodiment of the present invention, as illustrated in FIG. 21D,one frame period includes one writing period and one sensing period.Alternatively, as shown in FIG. 21E, two sensing periods may be includedin one frame period. When a plurality of detection periods are includedin one frame period, the detection sensitivity can be further increased.For example, two to four sensing periods may be included in one frameperiod.

Next, a structure example of the top surface of the sensor elementincluded in the touch panel 350D will be described with reference toFIGS. 22A to 22C.

FIG. 22A shows a top view of the sensor element. The sensor elementincludes a conductive layer 56 a and a conductive layer 56 b. Theconductive layer 56 a serves as one electrode of the sensor element, andthe conductive layer 56 b serves as the other electrode of the sensorelement. The sensor element can sense an approach or contact or the likeof an object utilizing the capacitance that is formed between theconductive layers 56 a and 56 b. Although not illustrated, theconductive layers 56 a and 56 b may have a top-surface shape that has acomb-like shape or that is provided with a slit.

In one embodiment of the present invention, the conductive layers 56 aand 56 b also serve as the common electrode of the liquid crystalelement.

A plurality of conductive layers 56 a are provided in the Y directionand extend in the X direction. A plurality of conductive layers 56 bprovided in the Y direction are electrically connected to each other viaa conductive layer 58 extending in the Y direction. FIG. 22A illustratesan example in which m conductive layers 56 a and n conductive layers 58are provided.

Note that the plurality of conductive layers 56 a may be provided in theX direction and in that case, may extend in the Y direction. Theplurality of conductive layers 56 b provided in the X direction may beelectrically connected to each other via the conductive layer 58extending in the X direction.

As illustrated in FIG. 22B, a conductive layer 56 serving as anelectrode of the sensor element is provided over a plurality of pixels60. The conductive layer 56 corresponds to each of the conductive layers56 a and 56 b in FIG. 22A. The pixel 60 is formed of a plurality ofsubpixels exhibiting different colors. FIG. 22B shows an example inwhich the pixel 60 is formed of three subpixels, subpixels 60 a, 60 b,and 60 c.

A pair of electrodes of the sensor element is preferably electricallyconnected to respective auxiliary wirings. The conductive layer 56 maybe electrically connected to an auxiliary wiring 57, as illustrated inFIG. 22C. Note that FIG. 22C illustrates an example in which theauxiliary wirings are stacked over the conductive layers; however, theconductive layers may be stacked over the auxiliary wirings. Theplurality of conductive layers 56 provided in the X direction may beelectrically connected to the conductive layer 58 through the auxiliarywiring 57.

The resistivity of the conductive layer that transmits visible light isrelatively high in some cases. Thus, the resistance of the pair ofelectrodes of the sensor element is preferably lowered by electricallyconnecting the pair of electrodes of the sensor element to the auxiliarywiring.

When the resistance of the pair of electrodes of the sensor element islowered, the time constant of the pair of electrodes can be small.Accordingly, the detection sensitivity of the sensor element can beincreased; furthermore, the detection accuracy of the sensor element canbe increased.

<1-8. Touch Panel Module>

Next, a touch panel module including the input/output device of oneembodiment of the present invention and an IC will be described withreference to FIG. 23 and FIGS. 24A to 24C.

FIG. 23 shows a block diagram of a touch panel module 6500. The touchpanel module 6500 includes a touch panel 6510 and an IC 6520. Theinput/output device of one embodiment of the present invention can beapplied to the touch panel 6510.

The touch panel 6510 includes a display portion 6511, an input portion6512, and a scan line driver circuit 6513. The display portion 6511includes a plurality of pixels, a plurality of signal lines, and aplurality of scan lines, and has a function of displaying an image. Theinput portion 6512 serves as a touch sensor by including a plurality ofsensor elements that can sense touch or proximity of a sensing target tothe touch panel 6510. A scan line driver circuit 6513 has a function ofoutputting a scan signal to the scan lines included in the displayportion 6511.

Here, the display portion 6511 and the input portion 6512 are separatelyillustrated as the components of the touch panel 6510 for simplicity;however, what is called an in-cell touch panel that has a function ofdisplaying an image and serves as a touch sensor is preferable.

The resolution of the display portion 6511 is preferably as high as HD(number of pixels: 1280×720), FHD (number of pixels: 1920×1080), WQHD(number of pixels: 2560×1440), WQXGA (number of pixels: 2560×1600), 4K(number of pixels: 3840×2160), or 8K (number of pixels: 7680×4320). Inparticular, resolution of 4K, 8K, or higher is preferable. The pixeldensity (definition) of the pixels in the display portion 6511 is higherthan or equal to 300 ppi, preferably higher than or equal to 500 ppi,more preferably higher than or equal to 800 ppi, more preferably higherthan or equal to 1000 ppi, and more preferably higher than or equal to1200 ppi. The display portion 6511 with such high resolution and highdefinition enables an increase in a realistic sensation, sense of depth,and the like in personal use such as portable use and home use.

The IC 6520 includes a circuit unit 6501, a signal line driver circuit6502, a sensor driver circuit 6503, and a detection circuit 6504. Thecircuit unit 6501 includes a timing controller 6505, an image processingcircuit 6506, and the like.

The signal line driver circuit 6502 has a function of outputting animage signal (a video signal) that is an analog signal to a signal lineincluded in the display portion 6511. For example, the signal linedriver circuit 6502 can include a shift register circuit and a buffercircuit in combination. The touch panel 6510 may include a demultiplexercircuit connected to a signal line.

The sensor driver circuit 6503 has a function of outputting a signal fordriving a sensor element included in the input portion 6512. As thesensor driver circuit 6503, a shift register circuit and a buffercircuit can be used in combination, for example.

The detection circuit 6504 has a function of outputting, to the circuitunit 6501, an output signal from the sensor element included in theinput portion 6512. The detection circuit 6504 can include an amplifiercircuit and an analog-digital converter (ADC), for example In that case,the detection circuit 6504 converts an analog signal output from theinput portion 6512 into a digital signal to be output to the circuitunit 6501.

The image processing circuit 6506 included in the circuit unit 6501 hasa function of generating and outputting a signal for driving the displayportion 6511 of the touch panel 6510, a function of generating andoutputting a signal for driving the input portion 6512, and a functionof analyzing a signal output from the input portion 6512 and outputtingthe signal to a CPU 6540.

As specific examples, the image processing circuit 6506 has thefollowing functions: a function of generating a video signal inaccordance with an instruction from the CPU 6540; a function ofperforming signal processing on a video signal in accordance with thespecification of the display portion 6511, converting the signal into ananalog video signal, and supplying the converted signal to the signalline driver circuit 6502; a function of generating a driving signaloutput to the sensor driver circuit 6503 in accordance with aninstruction from the CPU 6540; and a function of analyzing a signalinput from the detection circuit 6504 and outputting the analyzed signalto the CPU 6540 as positional information.

The timing controller 6505 has a function of generating a signal (e.g.,a clock signal or a start pulse signal) on the basis of asynchronization signal included in a video signal or the like on whichthe image processing circuit 6506 performs processing, and outputtingthe signal to the scan line driver circuit 6513 and the sensor drivercircuit 6503. Furthermore, the timing controller 6505 may have afunction of generating and outputting a signal for determining timingwhen the detection circuit 6504 outputs a signal. Here, the timingcontroller 6505 preferably outputs a signal synchronized with the signaloutput to the scan line driver circuit 6513 and a signal synchronizedwith the signal output to the sensor driver circuit 6503. In particular,it is preferable that a period in which data in a pixel in the displayportion 6511 is rewritten and a period in which sensing is performedwith the input portion 6512 be separately provided. For example, thetouch panel 6510 can be driven by dividing one frame period into aperiod in which data in a pixel is rewritten and a period in whichsensing is performed. Furthermore, detection sensitivity and detectionaccuracy can be increased, for example, by providing two or more sensingperiods in one frame period.

The image processing circuit 6506 can include a processor, for example.A microprocessor such as a digital signal processor (DSP) or a graphicsprocessing unit (GPU) can be used, for example. Furthermore, such amicroprocessor may be obtained with a programmable logic device (PLD)such as a field programmable gate array (FPGA) or a field programmableanalog array (FPAA). The image processing circuit 6506 interprets andexecutes instructions from various programs with the processor toprocess various kinds of data and control programs. The programsexecuted by the processor may be stored in a memory region included inthe processor or a memory device that is additionally provided.

A transistor that includes an oxide semiconductor in a channel formationregion and has an extremely low off-state current can be used in thedisplay portion 6511 or the scan line driver circuit 6513 included inthe touch panel 6510, the circuit unit 6501, the signal line drivercircuit 6502, the sensor driver circuit 6503, or the detection circuit6504 included in the IC 6520, the CPU 6540 provided outside, or thelike. With the use of the transistor having an extremely low off-statecurrent as a switch for holding electric charge (data) that flows into acapacitor serving as a memory element, a long data retention period canbe ensured. For example, by utilizing the characteristic for at leastone of a register and a cache memory of the image processing circuit6506, normally-off computing is achieved where the image processingcircuit 6506 operates only when needed and data on the previousprocessing is stored in the memory element in the rest of time; thus,the power consumption of the touch panel module 6500 and an electronicdevice on which the touch panel module 6500 is mounted can be reduced.

In this example, the circuit unit 6501 includes the timing controller6505 and the image processing circuit 6506; alternatively, the imageprocessing circuit 6506 itself or a circuit having a function of part ofthe image processing circuit 6506 may be provided outside.Alternatively, the CPU 6540 may have a function of the image processingcircuit 6506 or part thereof. For example, the circuit unit 6501 caninclude the signal line driver circuit 6502, the sensor driver circuit6503, the detection circuit 6504, and the timing controller 6505.

In this example, the IC 6520 includes the circuit unit 6501; the circuitunit 6501 is not necessarily included in the IC 6520. In that case, theIC 6520 can include the signal line driver circuit 6502, the sensordriver circuit 6503, and the detection circuit 6504. For example, in thecase where the touch panel module 6500 includes a plurality of ICs, thecircuit unit 6501 may be provided outside the touch panel module 6500and a plurality of ICs 6520 without the circuit unit 6501 may beprovided, and alternatively, the IC 6520 and an IC including only thesignal line driver circuit 6502 can be provided in combination.

When an IC has a function of driving the display portion 6511 of thetouch panel 6510 and a function of driving the input portion 6512 asdescribed above, the number of ICs mounted on the touch panel module6500 can be reduced; accordingly, cost can be reduced.

FIGS. 24A to 24C each are a schematic diagram of the touch panel module6500 on which the IC 6520 is mounted.

In FIG. 24A, the touch panel module 6500 includes a substrate 6531, acounter substrate 6532, a plurality of FPCs 6533, the IC 6520, ICs 6530,and the like. The touch panel module 6500 also includes the displayportion 6511, the input portion 6512, and the scan line driver circuit6513. The IC 6520 and the ICs 6530 are mounted on the substrate 6531 bya COG method or the like.

The IC 6530 is an IC in which only the signal line driver circuit 6502is provided in the above-described IC 6520 or an IC in which the signalline driver circuit 6502 and the circuit unit 6501 are provided in theabove-described IC 6520. The ICs 6520 and 6530 are supplied with asignal from the outside through the FPCs 6533. Furthermore, a signal canbe output to the outside from at least one of the ICs 6520 and 6530through the FPC 6533.

FIG. 24A illustrates an example where the display portion 6511 ispositioned between two scan line driver circuits 6513. The ICs 6530 areprovided in addition to the IC 6520. Such a structure is preferable inthe case where the display portion 6511 has extremely high resolution.

FIG. 24B illustrates an example where one IC 6520 and one FPC 6533 areprovided. It is preferable to bring functions into one IC 6520 in thismanner because the number of components can be reduced. In the examplein FIG. 24B, the scan line driver circuit 6513 is provided along a sideclose to the FPC 6533 among two short sides of the display portion 6511.

FIG. 24C illustrates an example of including a printed circuit board(PCB) 6534 on which the image processing circuit 6506 and the like aremounted. The ICs 6520 and 6530 over the substrate 6531 are electricallyconnected to the PCB 6534 through the FPCs 6533. Here, theabove-described structure without the image processing circuit 6506 canbe applied to the IC 6520.

In each of FIGS. 24A to 24C, the ICs 6520 and 6530 may be mounted on theFPC 6533, not on the substrate 6531. For example, the ICs 6520 and 6530can be mounted on the FPC 6533 by a COF method, a tape automated bonding(TAB) method, or the like.

A structure where the FPC 6533, the IC 6520 (and the IC 6530), and thelike are provided on a short side of the display portion 6511 asillustrated in FIGS. 24A and 24B enables the frame of the display deviceto be narrowed; thus, the structure is preferably used for electronicdevices such as smartphones, mobile phones, and tablet terminals, forexample. The structure with the PCB 6534 illustrated in FIG. 24C can bepreferably used for television devices, monitors, tablet terminals, ornotebook personal computers, for example.

As described above, the display device of one embodiment of the presentinvention includes the second common electrode on a substrate that facesa substrate on which the pixel electrode and the first common electrodeare provided. The same potential is supplied to the first and secondcommon electrodes, whereby light leakage can be prevented and thedisplay quality of the display device can be improved. Furthermore, thedisplay device can have a high aperture ratio and high definition. Inaddition, when the second common electrode is provided in part of adisplay region of a pixel, an increase in the driving voltage of theliquid crystal element can be prevented even when the second commonelectrode is provided.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, transistors that can be used for the display deviceof one embodiment of the present invention will be described withreference to FIGS. 25A to 35D.

The display device of one embodiment of the present invention can befabricated by using a transistor with any of various modes, such as abottom-gate transistor or a top-gate transistor. Therefore, a materialfor a semiconductor layer or the structure of a transistor can be easilychanged in accordance with the existing production line.

[Bottom-Gate Transistor]

FIG. 25A1 is a cross-sectional view of a transistor 410 that is achannel-protective transistor, which is a type of bottom-gatetransistor. The transistor 410 includes an electrode 546 over asubstrate 571 with an insulating layer 572 positioned therebetween. Thetransistor 410 includes a semiconductor layer 542 over the electrode 546with an insulating layer 526 positioned therebetween. The electrode 546can serve as a gate electrode. The insulating layer 526 can serve as agate insulating layer.

The transistor 410 includes an insulating layer 522 over a channelformation region in the semiconductor layer 542. The transistor 410includes an electrode 544 a and an electrode 544 b which are partly incontact with the semiconductor layer 542 and over the insulating layer526. Part of the electrode 544 a and part of the electrode 544 b areformed over the insulating layer 522.

The insulating layer 522 can serve a channel protective layer. With theinsulating layer 522 provided over the channel formation region, thesemiconductor layer 542 can be prevented from being exposed at the timeof forming the electrodes 544 a and 544 b. Thus, the channel formationregion in the semiconductor layer 542 can be prevented from being etchedat the time of forming the electrodes 544 a and 544 b. According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided.

The transistor 410 includes an insulating layer 528 over the electrode544 a, the electrode 544 b, and the insulating layer 522 and furtherincludes an insulating layer 529 over the insulating layer 528.

In the case where an oxide semiconductor is used for the semiconductorlayer 542, a material capable of removing oxygen from part of thesemiconductor layer 542 to generate oxygen vacancies is preferably usedfor regions of the electrodes 544 a and 544 b that are in contact withat least the semiconductor layer 542. The carrier concentrationincreases in the regions of the semiconductor layer 542 where oxygenvacancies are generated, so that the regions become n-type regions (nlayers). Accordingly, the regions can serve as a source region and adrain region. Examples of the material capable of removing oxygen fromthe oxide semiconductor to generate oxygen vacancies include tungstenand titanium.

Formation of the source region and the drain region in the semiconductorlayer 542 makes it possible to reduce the contact resistance between thesemiconductor layer 542 and each of the electrodes 544 a and 544 b.Accordingly, the electric characteristics of the transistor, such as thefield-effect mobility and the threshold voltage, can be favorable.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 542, a layer that serves as an n-type semiconductoror a p-type semiconductor is preferably provided between thesemiconductor layer 542 and the electrode 544 a and between thesemiconductor layer 542 and the electrode 544 b. The layer that servesas an n-type semiconductor or a p-type semiconductor can serve as thesource region or the drain region in the transistor.

The insulating layers 528 and 529 are preferably formed using a materialthat can prevent or reduce diffusion of impurities into the transistorfrom the outside. The insulating layer 529 is not necessarily formed.

When an oxide semiconductor is used for the semiconductor layer 542,heat treatment may be performed once or plural times before theinsulating layer 528 is formed, after the insulating layer 528 isformed, or after the insulating layer 529 is formed. The heat treatmentcan fill oxygen vacancies in the semiconductor layer 542 by diffusingoxygen contained in the insulating layers 528 and 529 or otherinsulating layers into the semiconductor layer 542. Alternatively, oneor both of the insulating layers 528 and 529 may be formed while theheat treatment is performed, so that oxygen vacancies in thesemiconductor layer 542 can be filled.

A transistor 411 illustrated in FIG. 25A2 is different from thetransistor 410 in that an electrode 523 that can serve as a back gate isprovided over the insulating layer 529. The electrode 523 can be formedusing a material and a method similar to those of the electrode 546.

<Back Gate>

In general, a back gate is formed using a conductive layer. The gate andthe back gate are positioned so that a channel formation region of asemiconductor layer is provided between the gate and the back gate. Theback gate can function in a manner similar to that of the gate. Thepotential of the back gate may be the same as that of the gate electrodeor may be a GND potential or a given potential. By changing thepotential of the back gate independently of the potential of the gate,the threshold voltage of the transistor can be changed.

The electrode 546 and the electrode 523 can each function as a gate.Thus, the insulating layers 526, 528, and 529 can each function as agate insulating layer. The electrode 523 may also be provided betweenthe insulating layers 528 and 529.

In the case where one of the electrodes 546 and 523 is simply referredto as a “gate” or a “gate electrode”, the other can be referred to as a“back gate” or a “back gate electrode”. For example, in the transistor411, in the case where the electrode 523 is referred to as a “gateelectrode”, the electrode 546 is referred to as a “back gate electrode”.In the case where the electrode 523 is used as a “gate electrode”, thetransistor 411 can be regarded as a kind of top-gate transistor.Alternatively, one of the electrodes 546 and 523 may be referred to as a“first gate” or a “first gate electrode”, and the other may be referredto as a “second gate” or a “second gate electrode”.

By providing the electrodes 546 and 523 with the semiconductor layer 542positioned therebetween and setting the potentials of the electrodes 546and 523 to be the same, a region of the semiconductor layer 542 throughwhich carriers flow is enlarged in the film thickness direction; thus,the number of transferred carriers is increased. As a result, theon-state current and the field-effect mobility of the transistor 411 areincreased.

Therefore, the transistor 411 has large on-state current for the areaoccupied thereby. That is, the area occupied by the transistor 411 canbe small for required on-state current. According to one embodiment ofthe present invention, the area occupied by a transistor can be reduced.Therefore, a display device can have a high aperture ratio or highdefinition.

Furthermore, the gate and the back gate are formed using conductivelayers and thus each have a function of preventing an electric fieldgenerated outside the transistor from influencing the semiconductorlayer in which the channel is formed (in particular, an electric fieldblocking function against static electricity and the like). When theback gate is formed larger than the semiconductor layer such that thesemiconductor layer is covered with the back gate, the electric fieldblocking function can be enhanced.

Since the electrode 546 (gate) and the electrode 523 (back gate) eachhave a function of blocking an electric field from the outside, electriccharge of charged particles and the like generated on the insulatinglayer 572 side or above the electrode 523 do not influence the channelformation region in the semiconductor layer 542. Thus, degradation by astress test (e.g., a negative gate bias temperature (−GBT) stress testin which negative charges are applied to a gate) can be reduced.Furthermore, a change in gate voltage (rising voltage) at which on-statecurrent starts flowing at different drain voltages can be reduced. Notethat this effect is obtained when the electrodes 546 and 523 have thesame potential or different potentials.

Note that the GBT stress test is an acceleration test and can evaluate,in a short time, a change by long-term use (i.e., a change over time) incharacteristics of a transistor. In particular, the amount of change inthe threshold voltage of the transistor between before and after the GBTstress test is an important indicator when examining the reliability ofthe transistor. As the change in threshold voltage is smaller, thetransistor has higher reliability.

By providing the electrodes 546 and 523 and setting the potentials ofthe electrodes 546 and 523 to be the same, the amount of change inthreshold voltage is reduced. Accordingly, a variation in electricalcharacteristics among a plurality of transistors is also reduced.

Also by a +GBT stress test in which positive electric charges areapplied to a gate, the transistor including the back gate has a smallerchange in threshold voltage than a transistor including no back gate.

When the back gate is formed using a light-blocking conductive film,light can be prevented from entering the semiconductor layer from theback gate side. Therefore, photodegradation of the semiconductor layercan be prevented and deterioration in electrical characteristics of thetransistor, such as a shift of the threshold voltage, can be prevented.

According to one embodiment of the present invention, highly reliabletransistor can be achieved. In addition, a highly reliable displaydevice can be achieved.

FIG. 25B1 is a cross-sectional view of a channel-protective transistor420 that is a type of bottom-gate transistor. The transistor 420 hassubstantially the same structure as the transistor 410 but is differentfrom the transistor 410 in that the insulating layer 522 having openings531 a and 531 b covers the semiconductor layer 542. The openings 531 aand 531 b are formed by selectively removing part of the insulatinglayer 522 which overlaps with the semiconductor layer 542.

The semiconductor layer 542 is electrically connected to the electrode544 a in the opening 531 a. The semiconductor layer 542 is electricallyconnected to the electrode 544 b in the opening 531 b. With theinsulating layer 522, the semiconductor layer 542 can be prevented frombeing exposed at the time of forming the electrodes 544 a and 544 b.Thus, the semiconductor layer 542 can be prevented from being reduced inthickness at the time of forming the electrodes 544 a and 544 b. Aregion of the insulating layer 522 that overlaps with the channelformation region can function as a channel protective layer.

A transistor 421 illustrated in FIG. 25B2 is different from thetransistor 420 in that the electrode 523 that can function as a backgate is provided over the insulating layer 529.

The distance between the electrodes 544 a and 546 and the distancebetween the electrodes 544 b and 546 in the transistors 420 and 421 arelonger than those in the transistors 410 and 411. Thus, the parasiticcapacitance generated between the electrodes 544 a and 546 can bereduced. Furthermore, the parasitic capacitance generated between theelectrodes 544 b and 546 can be reduced. According to one embodiment ofthe present invention, a transistor with favorable electricalcharacteristics can be achieved

A transistor 425 illustrated in FIG. 25C1 is a channel-etched transistorthat is a type of bottom-gate transistor. In the transistor 425, theinsulating layer 522 is not provided and the electrodes 544 a and 544 bare formed in contact with the semiconductor layer 542. Thus, part ofthe semiconductor layer 542 that is exposed when the electrodes 544 aand 544 b are formed is etched in some cases. However, since theinsulating layer 522 is not provided, the productivity of the transistorcan be increased.

A transistor 426 illustrated in FIG. 25C2 is different from thetransistor 425 in that the electrode 523 which can function as a backgate is provided over the insulating layer 529.

[Top-Gate Transistor]

FIG. 26A1 is a cross-sectional view of a transistor 430 that is a typeof top-gate transistor. The transistor 430 includes the semiconductorlayer 542 over the substrate 571 with the insulating layer 572positioned therebetween, the electrodes 544 a and 544 b that are overthe semiconductor layer 542 and the insulating layer 572 and in contactwith part of the semiconductor layer 542, the insulating layer 526 overthe semiconductor layer 542 and the electrodes 544 a and 544 b, and theelectrode 546 over the insulating layer 526.

Since the electrode 546 overlaps with neither the electrode 544 a northe electrode 544 b in the transistor 430, the parasitic capacitancegenerated between the electrodes 546 and 544 a and the parasiticcapacitance generated between the electrodes 546 and 544 b can bereduced. After the formation of the electrode 546, an impurity 555 isintroduced into the semiconductor layer 542 using the electrode 546 as amask, so that an impurity region can be formed in the semiconductorlayer 542 in a self-aligned manner (see FIG. 26A3). According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be achieved.

The introduction of the impurity 555 can be performed with an ionimplantation apparatus, an ion doping apparatus, or a plasma treatmentapparatus.

As the impurity 555, for example, at least one element of a Group 13element, a Group 15 element, and the like can be used. In the case wherean oxide semiconductor is used for the semiconductor layer 542, it ispossible to use at least one kind of element of a rare gas and hydrogenas the impurity 555.

A transistor 431 illustrated in FIG. 26A2 is different from thetransistor 430 in that the electrode 523 and an insulating layer 527 areincluded. The transistor 431 includes the electrode 523 formed over theinsulating layer 572 and the insulating layer 527 formed over theelectrode 523. The electrode 523 can function as a back gate. Thus, theinsulating layer 527 can function as a gate insulating layer. Theinsulating layer 527 can be formed using a material and a method similarto those of the insulating layer 526.

The transistor 431 as well as the transistor 411 has a high on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 431 can be small for required on-state current. According toone embodiment of the present invention, the area occupied by atransistor can be reduced. Therefore, according to one embodiment of thepresent invention, a display device can have a high aperture ratio orhigh definition.

A transistor 440 shown in FIG. 26B1 as an example is a type of top-gatetransistor. The transistor 440 is different from the transistor 430 inthat the semiconductor layer 542 is formed after the formation of theelectrodes 544 a and 544 b. A transistor 441 illustrated in FIG. 26B2 isdifferent from the transistor 440 in that the electrode 523 and theinsulating layer 527 are included. Thus, in the transistors 440 and 441,part of the semiconductor layer 542 is formed over the electrode 544 aand another part of the semiconductor layer 542 is formed over theelectrode 544 b.

The transistor 441 as well as the transistor 411 has a high on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 441 can be small for required on-state current. According toone embodiment of the present invention, the area occupied by atransistor can be reduced. Therefore, a display device can have a highaperture ratio or high definition.

A transistor 442 illustrated in FIG. 27A1 as an example is a type oftop-gate transistor. The transistor 442 has the electrodes 544 a and 544b over the insulating layer 529. The electrodes 544 a and 544 b areelectrically connected to the semiconductor layer 542 through openingsformed in the insulating layers 528 and 529.

Part of the insulating layer 526 that does not overlap with theelectrode 546 is removed. The insulating layer 526 included in thetransistor 442 partly extends across the ends of the electrode 546.

The impurity 555 is added to the semiconductor layer 542 using theelectrode 546 and the insulating layer 526 as masks, so that an impurityregion can be formed in the semiconductor layer 542 in a self-alignedmanner (see FIG. 27A3).

At this time, the impurity 555 is not added to the semiconductor layer542 in a region overlapping with the electrode 546, and the impurity 555is added to the semiconductor layer 542 in a region that does notoverlap with the electrode 546. The semiconductor layer 542 in a regioninto which the impurity 555 is introduced through the insulating layer526 has a lower impurity concentration than the semiconductor layer 542in a region into which the impurity 555 is introduced without throughthe insulating layer 526. Thus, a lightly doped drain (LDD) region isformed in the semiconductor layer 542 in a region adjacent to theelectrode 546.

A transistor 443 illustrated in FIG. 27A2 is different from thetransistor 442 in that the electrode 523 is provided under thesemiconductor layer 542. The electrode 523 and the semiconductor layer542 overlap with the insulating layer 572 positioned therebetween. Theelectrode 523 can function as a back gate electrode.

As in a transistor 444 illustrated in FIG. 27B1 and a transistor 445illustrated in FIG. 27B2, the insulating layer 526 in a region that doesnot overlap with the electrode 546 may be wholly removed. Alternatively,as in a transistor 446 illustrated in FIG. 27C1 and a transistor 447illustrated in FIG. 27C2, the insulating layer 526 except for theopenings may be left without being removed.

In the transistors 444 to 447, after the formation of the electrode 546,the impurity 555 is added to the semiconductor layer 542 using theelectrode 546 as a mask, so that an impurity region can be formed in thesemiconductor layer 542 in a self-aligned manner.

[s-Channel Transistor]

FIGS. 28A to 28C illustrate an example of a transistor including anoxide semiconductor for the semiconductor layer 542. FIG. 28A is a topview of a transistor 451. FIG. 28B is a cross-sectional view (in thechannel length direction) of a portion along the dashed-dotted lineL1-L2 in FIG. 28A. FIG. 28C is a cross-sectional view (in the channelwidth direction) of a portion along the dash-dotted line W1-W2 in FIG.28A.

The transistor 451 includes the semiconductor layer 542, the insulatinglayer 526, the insulating layer 572, an insulating layer 582, aninsulating layer 574, an electrode 524, an electrode 543, the electrode544 a, and the electrode 544 b. The electrode 543 can function as agate, and the electrode 524 can function as a back gate. The insulatinglayer 526, the insulating layer 572, the insulating layer 582, and theinsulating layer 574 each can function as a gate insulating layer. Theelectrode 544 a can function as one of a source electrode and a drainelectrode. The electrode 544 b can function as the other of the sourceelectrode and the drain electrode.

An insulating layer 575 is provided over the substrate 571, and theelectrode 524 and an insulating layer 573 are provided over theinsulating layer 575. Over the electrode 524 and the insulating layer573, the insulating layer 574 is provided. Over the insulating layer574, the insulating layer 582 is provided, and over the insulating layer582, the insulating layer 572 is provided.

A semiconductor layer 542 a is provided over a projection formed in theinsulating layer 572, and a semiconductor layer 542 b is provided overthe semiconductor layer 542 a. The electrode 544 a and the electrode 544b are provided over the semiconductor layer 542 b. A region of thesemiconductor layer 542 b that overlaps with the electrode 544 a canfunction as one of a source and a drain of the transistor 451. A regionof the semiconductor layer 542 b that overlaps with the electrode 544 bcan function as the other of the source and the drain of the transistor451.

In addition, a semiconductor layer 542 c is provided in contact withpart of the semiconductor layer 542 b. The insulating layer 526 isprovided over the semiconductor layer 542 c, and the electrode 543 isprovided over the insulating layer 526.

The transistor 451 has a structure in which a top surface and a sidesurface of the semiconductor layer 542 b and a side surface of thesemiconductor layer 542 a are covered with the semiconductor layer 542 cin FIG. 28C. With the semiconductor layer 542 b provided over theprojection of the insulating layer 572, the side surface of thesemiconductor layer 542 b can be covered with the electrode 543. Thatis, the transistor 451 has a structure in which the semiconductor layer542 b can be electrically surrounded by electric field of the electrode543. Such a structure of a transistor in which the semiconductor layerin which the channel is formed is electrically surrounded by theelectric field of the conductive film is called a surrounded channel(s-channel) structure. A transistor having an s-channel structure isreferred to as an s-channel transistor.

In the s-channel structure, a channel can be formed in the whole (bulk)of the semiconductor layer 542 b. In the s-channel structure, the draincurrent of the transistor is increased, so that a larger amount ofon-state current can be obtained. Furthermore, the entire channelformation region of the semiconductor layer 542 b can be depleted by theelectric field of the electrode 543. Accordingly, the off-state currentof the transistor with an s-channel structure can be further reduced.

When the projection of the insulating layer 572 is increased in height,and the channel width is shortened, the effects of the s-channelstructure for increasing the on-state current and reducing the off-statecurrent can be enhanced. Part of the semiconductor layer 542 a that isexposed in the formation of the semiconductor layer 542 b may beremoved. In this case, the side surfaces of the semiconductor layer 542a and the semiconductor layer 542 b may be aligned to each other.

The insulating layer 528 is provided over the transistor 451 and theinsulating layer 529 is provided over the insulating layer 528. Anelectrode 525 a, an electrode 525 b, and an electrode 525 c are providedover the insulating layer 529. The electrode 525 a is electricallyconnected to the electrode 544 a via a contact plug in an opening in theinsulating layer 529 and the insulating layer 528. The electrode 525 bis electrically connected to the electrode 544 b via a contact plug inan opening in the insulating layer 529 and the insulating layer 528. Theelectrode 525 c is electrically connected to an electrode 543 via acontact plug through an opening in the insulating layer 529 and theinsulating layer 528.

As the contact plug, for example, a conductive material with highembeddability, such as tungsten or polysilicon, can be used. A sidesurface and a bottom surface of the material may be covered with abarrier layer (a diffusion prevention layer) of a titanium layer, atitanium nitride layer, or a stacked layer of these layers. In thiscase, the barrier layer is regarded as part of the contact plug in somecases.

Note that when the insulating layer 582 is formed using hafnium oxide,aluminum oxide, tantalum oxide, aluminum silicate, or the like, theinsulating layer 582 can function as a charge trap layer. The thresholdvoltage of the transistor can be changed by injecting electrons into theinsulating layer 582. For example, the injection of electrons into theinsulating layer 582 can be performed with use of the tunnel effect. Byapplying a positive voltage to the electrode 524, tunnel electrons canbe injected into the insulating layer 582.

The electrode 524 that can function as a back gate is not necessarilyprovided, depending on the purpose. FIG. 29A is a top view of atransistor 451 a. FIG. 29B is a cross-sectional view along dashed-dottedline L1-L2 in FIG. 29A, and FIG. 29C is a cross-sectional view alongdashed-dotted line W1-W2 in FIG. 29A. The transistor 451 a has astructure in which the electrode 524, and the insulating layers 573,574, and 582 are removed from the transistor 451. The productivity ofthe transistor can be improved by omission of the electrode and theinsulating layers. Accordingly, the productivity of the display devicecan be improved.

FIGS. 30A to 30C illustrate another example of an s-channel transistor.FIG. 30A is a top view of a transistor 452. FIG. 30B is across-sectional view along dashed-dotted line L1-L2 in FIG. 30A. FIG.30C is a cross-sectional view along dashed-dotted line W1-W2 in FIG.30A.

The transistor 452 has the same structure as the transistor 451, exceptin that the electrode 544 a and the electrode 544 b are in contact withthe side surfaces of the semiconductor layer 542 a and the semiconductorlayer 542 b. As the insulating layer 528 covering the transistor 452, aninsulating layer with a flat surface such as that in the transistor 451may be used. In addition, the electrode 525 a, the electrode 525 b, andthe electrode 525 c may be provided over the insulating layer 529.

FIGS. 31A and 31B illustrate another example of an s-channel transistor.FIG. 31A is a top view of a transistor 453. FIG. 31B is across-sectional view along dashed-dotted line L1-L2 and dashed-dottedline W1-W2 in FIG. 31A. As in the transistor 451, the transistor 453includes the semiconductor layer 542 a and the semiconductor layer 542 bover the projection of the insulating layer 572. The electrode 544 a andthe electrode 544 b are provided over the semiconductor layer 542 b. Aregion of the semiconductor layer 542 b that overlaps with the electrode544 a can function as one of a source and a drain of the transistor 453.A region of the semiconductor layer 542 b that overlaps with theelectrode 544 b can function as the other of the source and the drain ofthe transistor 453. Thus, a region 569 of the semiconductor layer 542 bthat is located between the electrode 544 a and the electrode 544 b canfunction as a channel formation region.

In the transistor 453, an opening is provided in a region overlappingwith the region 569 by removing part of the insulating layer 528, andthe semiconductor layer 542 c is provided along a side and bottomsurfaces of the opening. In the opening, the insulating layer 526 isprovided along the side and bottom surfaces of the opening with thesemiconductor layer 542 c located therebetween. In addition, in theopening, the electrode 543 is provided along the side and bottomsurfaces of the opening with the semiconductor layer 542 c and theinsulating layer 526 located therebetween.

Note that the opening is wider than the semiconductor layer 542 a andthe semiconductor layer 542 b in the cross section in the channel widthdirection. Thus, in the region 569, side surfaces of the semiconductorlayer 542 a and the semiconductor layer 542 b are covered with thesemiconductor layer 542 c.

The insulating layer 529 is provided over the insulating layer 528 andan insulating layer 577 is provided over the insulating layer 529. Theelectrode 525 a, the electrode 525 b, and the electrode 525 c areprovided over the insulating layer 577. The electrode 525 a iselectrically connected to the electrode 544 a via a contact plug in anopening formed by removing part of the insulating layers 577, 529, and528. The electrode 525 b is electrically connected to the electrode 544b via a contact plug in an opening formed by removing part of theinsulating layers 577, 529, and 528. The electrode 525 c is electricallyconnected to the electrode 543 via a contact plug in an opening formedby removing part of the insulating layers 577 and 529.

The electrode 524 that can function as a back gate is not necessarilyprovided, depending on the purpose. FIG. 32A is a top view of atransistor 453 a. FIG. 32B is a cross-sectional view along dashed-dottedline L1-L2 and dashed-dotted line W1-W2 in in FIG. 32A. The transistor453 a has a structure in which the electrode 524, and the insulatinglayers 574 and 582 are removed from the transistor 453. The productivityof the transistor can be improved by omission of the electrode and theinsulating layers. Accordingly, the productivity of the display devicecan be improved.

FIGS. 33A to 33C illustrate another example of an s-channel transistor.FIG. 33A is a top view of a transistor 454. FIG. 33B is across-sectional view along dashed-dotted line L1-L2 in FIG. 33A. FIG.33C is a cross-sectional view along dashed-dotted line W1-W2 in FIG.33A.

The transistor 454 is a kind of bottom-gate transistor having aback-gate electrode. In the transistor 454, the electrode 543 is formedover the insulating layer 574, and the insulating layer 526 is providedto cover the electrode 543. The semiconductor layer 542 is formed in aregion that is over the insulating layer 526 and overlaps with theelectrode 543. The semiconductor layer 542 in the transistor 454 has astacked structure of the semiconductor layer 542 a and the semiconductorlayer 542 b.

The electrode 544 a and the electrode 544 b are formed over theinsulating layer 526 so as to be in contact with part of thesemiconductor layer 542. The insulating layer 528 is formed over theelectrode 544 a and the electrode 544 b so as to be in contact with partof the semiconductor layer 542. The insulating layer 529 is formed overthe insulating layer 528. The electrode 524 is formed in a region overthe insulating layer 529 that overlaps with the semiconductor layer 542.

The electrode 524 provided over the insulating layer 529 is electricallyconnected to the electrode 543 in an opening 547 a and an opening 547 bprovided in the insulating layer 529, the insulating layer 528, and theinsulating layer 526. Accordingly, the same potential is supplied to theelectrodes 524 and 543. Furthermore, either or both of the openings 547a and 547 b may be omitted. In the case where neither the opening 547 anor the opening 547 b is provided, different potentials can be suppliedto the electrode 524 and the electrode 543.

The electrode 524 that can function as a back gate is not necessarilyprovided, depending on the purpose. FIG. 34A is a top view of atransistor 454 a. FIG. 34B is a cross-sectional view along dashed-dottedline L1-L2 in FIG. 34A. FIG. 34C is a cross-sectional view alongdashed-dotted line W1-W2 in in FIG. 34A. The transistor 454 a has astructure in which the electrode 524, and the openings 547 a and 547 bare removed from the transistor 454. The productivity of the transistorcan be improved by omission of the electrode and the openings.Accordingly, the productivity of the display device can be improved.

FIGS. 35A to 35C illustrate another example of an s-channel transistor.A transistor 448 in FIGS. 35A to 35C has almost the same structure asthe transistor 447. The transistor 448 is a kind of top-gate transistorhaving a back gate. FIG. 35A is a top view of the transistor 448. FIG.35B is a cross-sectional view along dashed-dotted line L1-L2 in FIG.35A. FIG. 35C is a cross-sectional view along dashed-dotted line W1-W2in FIG. 35A.

FIGS. 35A to 35C illustrate an example in which an inorganicsemiconductor layer such as a silicon layer is used as the semiconductorlayer 542 in the transistor 448. In FIGS. 35A to 35C, the electrode 524is provided over the substrate 571, and the insulating layer 572 isprovided over the electrode 524. In addition, the semiconductor layer542 is formed over a projection of the insulating layer 572.

The semiconductor layer 542 includes a semiconductor layer 542 i, twosemiconductor layers 542 t, and two semiconductor layers 542 u. Thesemiconductor layer 542 i is sandwiched between the two semiconductorlayers 542 t. The semiconductor layer 542 i and the two semiconductorlayers 542 t are sandwiched between the two semiconductor layers 542 u.The electrode 543 is provided in a region overlapping with thesemiconductor layer 542 i.

A channel is formed in the semiconductor layer 542 i when the transistor448 is on. Therefore, the semiconductor layer 542 i serves as a channelformation region. The semiconductor layers 542 t serve as lowconcentration impurity regions (i.e., LDD regions). The semiconductorlayers 542 u serve as high concentration impurity regions. Note that oneor both of the two semiconductor layers 542 t are not necessarilyprovided. One of the two semiconductor layers 542 u serves as a sourceregion, and the other semiconductor layer 542 u serves as a drainregion.

The electrode 544 a provided over the insulating layer 529 iselectrically connected to one of the semiconductor layers 542 u in anopening 547 c formed in the insulating layers 526, 528, and 529. Theelectrode 544 b provided over the insulating layer 529 is electricallyconnected to the other of the semiconductor layers 542 u in an opening547 d formed in the insulating layers 526, 528, and 529.

The electrode 543 provided over the insulating layer 526 is electricallyconnected to the electrode 524 in the opening 547 a and the opening 547b formed in the insulating layers 526 and 572. Accordingly, the samepotential is supplied to the electrodes 543 and 524. Furthermore, eitheror both of the openings 547 a and 547 b may be omitted. In the casewhere neither the opening 547 a nor the opening 547 b is provided,different potentials can be applied to the electrodes 524 and 543.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, a touch panel module and electronic devices thatinclude the display device of one embodiment of the present inventionwill be described with reference to FIG. 36 , FIGS. 37A to 37H, andFIGS. 38A and 38B.

In a touch panel module 8000 illustrated in FIG. 36 , a touch panel 8004connected to an FPC 8003, a frame 8009, a printed circuit board 8010,and a battery 8011 are provided between a top cover 8001 and a bottomcover 8002.

The display device of one embodiment of the present invention can beused for the touch panel 8004, for example.

The shapes and sizes of the top cover 8001 and the bottom cover 8002 canbe changed as appropriate in accordance with the size of the touch panel8004.

The display device of one embodiment of the present invention canfunction as a touch panel. The touch panel 8004 can be a resistive touchpanel or a capacitive touch panel and can be formed to overlap with thedisplay device of one embodiment of the present invention. A countersubstrate (sealing substrate) of the touch panel 8004 can have a touchpanel function. A photo sensor may be provided in each pixel of thetouch panel 8004 so that an optical touch panel can be obtained.

When a transmissive liquid crystal element is used, a backlight 8007 maybe provided as illustrated in FIG. 36 . The backlight 8007 includes alight source 8008. Although the light sources 8008 are provided over thebacklight 8007 in FIG. 36 , one embodiment of the present invention isnot limited to this structure. For example, a structure in which thelight source 8008 is provided at an end portion of the backlight 8007and a light diffusion plate is further provided may be employed. In thecase where a self-luminous light-emitting element such as an organic ELelement is used or the case where a reflective panel or the like isused, the backlight 8007 is not necessarily provided.

The frame 8009 protects the touch panel 8004 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed circuit board 8010. The frame 8009 can alsofunction as a radiator plate.

The printed circuit board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The touch panel 8004 can be additionally provided with a component suchas a polarizer, a retardation film, or a prism sheet.

FIGS. 37A to 37H and FIGS. 38A and 38B illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or an operation switch), a connection terminal 5006, asensor 5007 (sensor having a function of measuring force,disarrangement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, smell, or infrared ray), a microphone 5008, and the like.

FIG. 37A illustrates a mobile computer, which can include a switch 5009,an infrared port 5010, and the like in addition to the above components.FIG. 37B illustrates a portable image reproducing device provided with amemory medium (e.g., a DVD reproducing device), which can include asecond display portion 5002, a memory medium reading portion 5011, andthe like in addition to the above components. FIG. 37C illustrates atelevision device, which can include a stand 5012 and the like inaddition to the above components. The television device can be operatedby an operation switch of the housing 5000 or a separate remotecontroller 5013. With operation keys of the remote controller 5013,channels and volume can be controlled, and images displayed on thedisplay portion 5001 can be controlled. The remote controller 5013 maybe provided with a display portion for displaying data output from theremote controller 5013. FIG. 37D illustrates a portable game machinewhich can include the memory medium reading portion 5011 and the like inaddition to the above components. FIG. 37E illustrates a digital camerahaving a television reception function, which can include an antenna5014, a shutter button 5015, an image receiving portion 5016, and thelike in addition to the above components. FIG. 37F illustrates aportable game machine which can include the second display portion 5002,the memory medium reading portion 5011, and the like in addition to theabove components. FIG. 37G illustrates a portable television receiverwhich can include a charger 5017 capable of transmitting and receivingsignals, and the like in addition to the above components. FIG. 37Hillustrates a wrist-watch-type information terminal, which can include aband 5018, a clasp 5019, and the like in addition to the abovecomponents. The display portion 5001 mounted in the housing 5000 alsoserving as a bezel includes a non-rectangular display region. Thedisplay portion 5001 can display an icon 5020 indicating time, anothericon 5021, and the like. FIG. 38A illustrates a digital signage. FIG.38B illustrates a digital signage mounted on a cylindrical pillar.

The electronic devices illustrated in FIGS. 37A to 37H and FIGS. 38A and38B can have a variety of functions. For example, the electronic devicesillustrated in FIGS. 37A to 37H and FIGS. 38A and 38B can have a varietyof functions, for example, a function of displaying a variety ofinformation (a still image, a moving image, a text image, and the like)on the display portion, a touch panel function, a function of displayinga calendar, the date, the time, and the like, a function of controllingprocessing with a variety of software (programs), a wirelesscommunication function, a function of connecting to a variety ofcomputer networks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading a program or datastored in a storage medium and displaying the program or data on thedisplay portion. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imageinformation mainly on one display portion while displaying textinformation on another display portion, a function of displaying athree-dimensional image by displaying images where parallax isconsidered on a plurality of display portions, or the like. Furthermore,the electronic device including an image receiver can have a function ofshooting a still image, a function of taking a moving image, a functionof automatically or manually correcting a shot image, a function ofstoring a shot image in a memory medium (an external memory medium or amemory medium incorporated in the camera), a function of displaying ashot image on the display area, or the like. Note that the functions ofthe electronic devices illustrated in FIGS. 37A to 37H and FIGS. 38A and38B are not limited thereto, and the electronic devices can have avariety of functions.

The electronic devices in this embodiment each include a display portionfor displaying some kind of information. The display device of oneembodiment of the present invention can be used for the display portion.

This embodiment can be combined with any of the other embodiments asappropriate.

Example 1

In this example, a liquid crystal display device of one embodiment ofthe present invention will be described.

<Examination of Semiconductor Materials>

In this example, an oxide semiconductor, specifically, a CAAC-OS is usedfor a semiconductor layer of a transistor.

A transistor using a CAAC-OS (a CAAC-OS FET) has a lower off-statecurrent than a transistor using low-temperature polysilicon (LTPS) (anLTPS FET).

In a non-selection period after data is written, charge graduallydecreases when an off-state current flows between a source and a drainof a selection transistor in a pixel. This changes the voltage appliedto liquid crystal molecules and causes a change in opticalcharacteristics to be visible. Hence, a display device with a highoff-state current requires constant data writing, leading to an increasein power consumption. The CAAC-OS FET has a lower off-state current thanthe LTPS FET; thus, almost no charge moves in a non-selection period andthe voltage applied to liquid crystals does not change. Therefore, powerconsumption can be prevented from increasing with the number of times ofrewriting.

<Examination of Transistor Structures>

In this example, two types of 1058 ppi pixel layouts with an FFS modewere fabricated: one uses a bottom-gate top-contact (BGTC) transistor,and the other uses a top-gate self-aligned (TGSA) transistor. Then, analignment simulation in the FFS mode was conducted on the twostructures.

FIGS. 39A and 39B illustrate pixel layouts using the BGTC transistor.FIG. 39A illustrates the transistor, the pixel electrode 111, and thefirst common electrode 112. The BGTC transistor includes the gate 221,the semiconductor layer 231, and the conductive layers 222 a and 222 bserving as the source and drain electrodes. FIG. 39B is a top view inwhich the first common electrode 112 is omitted from the layeredstructure in FIG. 39A.

In FIGS. 39A and 39B, one conductive layer can be considered to serve asboth the scan line 228 and the gate 221. Also in FIGS. 39A and 39B, oneconductive layer can be considered to serve as both the signal line 229and the conductive layer 222 a.

The pixel layouts with the TGSA transistor are similar to those in FIGS.3B and 3C.

In this example, design simulator for liquid crystal display devices(LCD Master 3-D Full set FEM mode) manufactured by Shintech, Inc. wasused, and a periodic boundary condition was adopted. The simulation inthis example was conducted on a structure including two adjacentsubpixels: two subpixels illustrated in FIG. 3B or FIG. 39A are arrangedside by side, the left subpixel displays white (a voltage ranging from 0V to 6 V is applied to the pixel electrode 111), and the right subpixeldisplays black (a voltage of 0 V is applied to the pixel electrode 111).Each subpixel has a size of 8 μm×24 μm.

The simulation was performed under the conditions where a negativeliquid crystal material (Δε=−3) is used, the cell gap is 3.5 μm, and avoltage of 0 V is applied to the first common electrode 112.

FIGS. 40A and 40B show the alignment simulation results with the BGTCtransistor and the TGSA transistor, respectively. In each of FIGS. 40Aand 40B, the in-plane distribution at the maximum transmittance isshown.

The alignment simulation results indicate that with the TGSA structure,a higher aperture ratio, liquid crystal transmittance, and effectivetransmittance can be obtained than with the BGTC structure.Specifically, the aperture ratio of the TGSA structure is 37.0%, 1.016times as high as that (36.4%) of the BGTC structure; the liquid crystaltransmittance of the TGSA structure is 1.030 times as high as that ofthe BGTC structure; and the effective transmittance of the TGSAstructure is 1.044 times as high as that of the BGTC structure.

In view of the above results, the TGSA transistors were used in thesubsequent examinations. In the following simulations in this example,two subpixels illustrated in FIG. 3B are arranged side by side, one ofwhich on the left displays white and the other of which on the rightdisplays black.

<Examination of Liquid Crystal Materials>

Next, an alignment simulation was conducted to compare the alignmentstates of a positive liquid crystal material (Δε=3.8) and a negativeliquid crystal material (Δε=−3).

FIGS. 41A and 41B show the alignment simulating results with thepositive liquid crystal material and the negative liquid crystalmaterial, respectively. In each of FIGS. 41A and 41B, the in-planedistribution at the maximum transmittance is shown.

The simulation was performed under the conditions where the cell gap is3.5 μm, a positive polarity is applied, and a flexoelectric effect isproduced. The flexoelectric effect is a phenomenon in which polarizationis induced by the distortion of orientation, and mainly depends on theshape of a molecule. The deformation causing the flexoelectric effectcan be reduced in the negative liquid crystal material than in thepositive liquid crystal material. The subsequent simulations in thisexample were all conducted under the condition with the flexoelectriceffect.

As shown in FIG. 41A, with the positive liquid crystal material, aregion with a lower transmittance due to alignment defects is found inthe subpixel displaying white. In addition, light leakage occurs in theadjacent subpixel (the subpixel displaying black).

As shown in FIG. 41B, with the negative liquid crystal material, thewhite subpixel is entirely covered by a transmitting region.Furthermore, the amount of light leakage observed in the periphery ofthe adjacent subpixel (black subpixel) is small compared with the casewhere the positive liquid crystal material is used.

In view of the above results, the negative liquid crystal materials wereused in the subsequent examinations.

Next, the results of the alignment simulation in which a negative liquidcrystal material is used and a positive or negative polarity is appliedwere compared. When the positive polarity is applied, the simulation wasperformed under the condition where a voltage of 0 V to 6 V is appliedto the pixel electrode 111 of the subpixel that displays white (the leftsubpixel); when the negative polarity is applied, the simulation wasperformed under the condition where a voltage of 0 V to −6 V is appliedto the pixel electrode 111 of the subpixel that displays white.

In this example, the alignment simulation was conducted under twoconditions: in a first condition, the cell gap is 3.5 μm; and in asecond condition, the cell gap is 2.5 μm and the second common electrode(to which a voltage of 0 V was applied) is employed. The layout of thesecond common electrode is similar to that of the first common electrode112 in FIG. 3B. That is, the first common electrode 112 and the secondcommon electrode include openings that have the same size and are in thesame position. The width of the opening (the horizontal length of theopening in the first common electrode 112 illustrated in FIG. 3B) is 3μm.

FIGS. 42A and 42B show the alignment simulation results where the cellgap is 3.5 μm. FIGS. 43A and 43B show the alignment simulation resultswhere the cell gap is 2.5 μm and the second common electrode (to which avoltage of 0 V is applied) is employed. In each of FIGS. 42A to 43B, thein-plane distribution at the maximum transmittance is shown. FIGS. 42Aand 43A show the results where a positive voltage is applied whereasFIGS. 42B and 43B show the results where a negative voltage is applied.

FIGS. 43A and 43B indicate that alignment defects in adjacent pixels canbe reduced by reducing the cell gap to 2.5 μm and using the secondcommon electrode. In addition, the distribution of the transmittance ofthe subpixel displaying white and the degree of light leakage betweenadjacent pixels do not vary with a difference in polarity. A variationin optical characteristics due to polarity is small, which suppressesflickering in the display device. In addition, a large light-blockingregion is not necessary because the amount of light leakage is small,which achieves a high aperture ratio.

In this example, the aperture ratio of the pixel layout in FIG. 39A(without the second common electrode) is 36.4%, and the aperture ratioof the pixel layout in FIG. 3B (without the second common electrode) is37.0%. By employing the second common electrode in the pixel layout inFIG. 3B, the aperture ratio increases to 41.0%.

Next, the voltage-transmittance (V-T) characteristics of a pixel areexamined by simulation. The dielectric anisotropy (Δε) is −3, −5, and−7. FIG. 44 shows the simulation result.

FIG. 44 indicates that the saturated voltage decreases as the absolutevalue of Δε increases and that the curve of Δε=−7 has maximumtransmittance at approximately 4 V.

<Fabrication of Liquid Crystal Display Device>

Based on the above simulation results, a transmissive liquid crystaldisplay device was fabricated by combining the pixel layout employingthe second common electrode with a negative liquid crystal material.

The specifications of the display device are as follows. The size of adisplay portion is 4.16 inches in diagonal, the number of effectivepixels is 3840 (H)×RGB×2160 (V), the definition is 1058 ppi, and thesize of a subpixel is 8 (H)×24 μm (V).

As the display element, a liquid crystal element with an FFS mode wasused. As the liquid crystal material, a negative liquid crystal materialwas used. A color filter (CF) method was used as the coloring method.The drive frequency was 60 Hz. An analog line sequential video signalformat was used as the video signal format. The gate driver wasincorporated. An analog switch was incorporated into the source driverand a COG was used.

A spacer with a height of approximately 2.5 μm was provided in thedisplay device, so that the cell gap was approximately 2.5 μm. Thedielectric anisotropy (Δε) of a liquid crystal was −8 and the refractiveindex anisotropy (Δn) of the liquid crystal was 0.118. The width of anopening in the second common electrode was approximately 3 μm and thedistance between openings in the second common electrode wasapproximately 5 μm.

FIG. 45A is a photograph of the display device fabricated in thisexample that is displaying an image. FIGS. 45B and 45C are opticalmicrographs of the display portion: white is displayed in FIG. 45B andgreen is displayed in FIG. 45C.

As shown in FIG. 45B, favorable alignment was found when the pixeldisplays white. As shown in FIG. 45C, light leakage from subpixels otherthan subpixels emitting green was found to be reduced when the pixeldisplays green.

By combining a negative liquid crystal material, which has advantages offavorable alignment and low-voltage driving, with a top-gate CAAC-OSFET, which has advantages of low power consumption, high aperture ratio,and high transmittance, a 4K liquid crystal display device with a highdefinition of over 1000 ppi was fabricated.

Example 2

In Example 1, as a condition of the alignment simulation in which anegative liquid crystal material is used and a positive or negativepolarity is applied, the cell gap is set to 2.5 μm and the second commonelectrode (to which a voltage of 0 V is applied) is employed.

This example shows the results of alignment simulations that are focusedon the cell gap and the width of an opening in the second commonelectrode.

In this example, design simulator for liquid crystal display devices(LCD Master 3-D Full set FEM mode) manufactured by Shintech, Inc. wasused, and a periodic boundary condition was adopted. The simulation inthis example was conducted on a structure including two adjacentsubpixels: two subpixels illustrated in FIG. 3B are arranged side byside, the left subpixel displays white (a voltage ranging from 0 V to 6V is applied to the pixel electrode 111), and the right subpixeldisplays black (a voltage of 0 V is applied to the pixel electrode 111).Each subpixel has a size of 8 μm×24 μm. The width of the opening (thehorizontal length of the opening in the first common electrode 112illustrated in FIG. 3B) is 3 μm.

The simulation was performed under the conditions where a negativeliquid crystal material (Δε=−3) is used, and a voltage of 0 V is appliedto the first common electrode 112 and the second common electrode.

First, an alignment simulation was conducted under five conditions: thewidth of the opening in the second common electrode is 2 μm, 3 μm, 4 μm,5 μm, and 8 μm. The width of the opening in the second common electrodeis equal to the length L1, which is illustrated in FIGS. 1A and 1B anddenotes the length of a region where the second common electrode 244 isnot provided. Each subpixel has a size of 8 μm×24 μm as described above;the condition with L1=8 μm corresponds to the condition where the secondcommon electrode is not provided in the subpixel. When L1=3 μm, thelayout of the second common electrode can be considered similar to thatof the first common electrode 112 illustrated in FIG. 3B. Note that thecell gap is 3 μm.

In this example, the transmittance and the contrast are calculated bythe alignment simulation. The transmittance here refers to the averagetransmittance of subpixels that display white. The contrast is obtainedby dividing the average transmittance of subpixels that display white bythe average transmittance of subpixels that display black.

FIG. 46A shows the simulation results of voltage-transmittancecharacteristics and FIG. 46B shows the simulation results oftransmittance-contrast characteristics. There results indicate that withthe same transmittance, the contrast increases as the opening of thesecond common electrode has a narrower width. It is also found that asthe opening of the second common electrode has a larger width, themaximum transmittance is obtained at a lower voltage.

Then, an alignment simulation was conducted under three conditions: thecell gap is 2.5 μm, 2.75 μm, and 3 μm. Note that the width of eachopening in the first common electrode and the second common electrode is3 μm.

FIG. 47A shows the simulation results of voltage-transmittancecharacteristics and FIG. 47B shows the simulation results oftransmittance-contrast characteristics. There results indicate that thecontrast increases as the cell gap decreases, and that the transmittanceincreases as the cell gap increases.

This application is based on Japanese Patent Application serial No.2016-050824 filed with Japan Patent Office on Mar. 15, 2016, andJapanese Patent Application serial No. 2016-101543 filed with JapanPatent Office on May 20, 2016, the entire contents of which are herebyincorporated by reference.

EXPLANATION OF REFERENCE

34: capacitor 40: liquid crystal element 45: light 51: substrate 56:conductive layer 56 a: conductive layer 56 b: conductive layer 57:auxiliary wiring 58: conductive layer 60: pixel 60 a: subpixel 60 b:subpixel 60 c: subpixel 61: substrate 62: display portion 63: connectionportion 64: driver circuit portion 65: wiring 66: non-display region 68:display region 68 a: display region 68 b: display region 69: connectionportion 72: FPC 72 a: FPC 72 b: FPC 73: IC 73 a: IC 73 b: IC 81: scanline 82: signal line 100A: display device 100B: display device 100C:display device 100D: display device 100E: display device 100F: displaydevice 111: pixel electrode 111 a: pixel electrode 111 b: pixelelectrode 112: first common electrode 112 a: first common electrode 112b: first common electrode 113: liquid crystal layer 117: spacer 119 a:substrate 119 b: substrate 121: overcoat 122: insulating layer 123:insulating layer 124: electrode 125: insulating layer 126: conductivelayer 127: electrode 128: electrode 130: polarizer 131: coloring layer132: light-blocking layer 132 a: light-blocking layer 132 b:light-blocking layer 133 a: alignment film 133 b: alignment film 137:wiring 138: wiring 139: auxiliary wiring 141: adhesive layer 160:protection substrate 161: backlight 162: substrate 163: adhesive layer164: adhesive layer 165: polarizer 166: polarizer 167: adhesive layer168: adhesive layer 169: adhesive layer 201: transistor 204: connectionportion 206: transistor 211: insulating layer 212: insulating layer 213:insulating layer 214: insulating layer 215: insulating layer 216:insulating layer 220: insulating layer 221: gate 222 a: conductive layer222 b: conductive layer 223: gate 228: scan line 229: signal line 231:semiconductor layer 231 a: channel region 231 b: low-resistance region242: connector 242 b: connector 243: connector 244: second commonelectrode 244 a: second common electrode 244 b: second common electrode244 c: second common electrode 251: conductive layer 281: conductivelayer 282: conductive layer 283: conductive layer 284: conductive layer285: conductive layer 286: conductive layer 350A: touch panel 350B:touch panel 350D: touch panel 360: region 370: display device 375: inputdevice 376: input device 379: display device 410: transistor 411:transistor 415: input device 416: substrate 420: transistor 421:transistor 425: transistor 426: transistor 430: transistor 431:transistor 440: transistor 441: transistor 442: transistor 443:transistor 444: transistor 445: transistor 446: transistor 447:transistor 448: transistor 449: IC 450: FPC 451: transistor 451 a:transistor 452: transistor 453: transistor 453 a: transistor 454:transistor 454 a: transistor 461: conductive film 462: conductive film463: conductive film 464: nanowire 471: electrode 472: electrode 473:electrode 474: bridge electrode 476: wiring 477: wiring 522: insulatinglayer 523: electrode 524: electrode 524 a: electrode 524 b: electrode525 a: electrode 525 b: electrode 525 c: electrode 526: insulating layer527: insulating layer 528: insulating layer 529: insulating layer 531 a:opening 531 b: opening 542: semiconductor layer 542 a: semiconductorlayer 542 b: semiconductor layer 542 c: semiconductor layer 542 i:semiconductor layer 542 t: semiconductor layer 542 u: semiconductorlayer 543: electrode 544 a: electrode 544 b: electrode 546: electrode547 a: opening 547 b: opening 547 c: opening 547 d: opening 555:impurity 569: region 571: substrate 572: insulating layer 573:insulating layer 574: insulating layer 575: insulating layer 577:insulating layer 582: insulating layer 601: pulse voltage output circuit602: current sensing circuit 603: capacitor 621: electrode 622:electrode 3501: wiring 3502: wiring 3510: wiring 3511: wiring 3515_1:block 3515_2: block 3516: block 5000: housing 5001: display portion5002: display portion 5003: speaker 5004: LED lamp 5005: operation key5006: connection terminal 5007: sensor 5008: microphone 5009: switch5010: infrared port 5011: memory medium reading portion 5012: stand5013: remote controller 5014: antenna 5015: shutter button 5016: imagereceiving portion 5017: charger 5018: band 5019: clasp 5020: icon 5021:icon 6500: touch panel module 6501: circuit unit 6502: signal linedriver circuit 6503: sensor driver circuit 6504: detection circuit 6505:timing controller 6506: image processing circuit 6510: touch panel 6511:display portion 6512: input portion 6513: scan line driver circuit 6520:IC 6530: IC 6531: substrate 6532: counter substrate 6533: FPC 6534: PCB6540: CPU 8000: touch panel module 8001: top cover 8002: bottom cover8003: FPC 8004: touch panel 8007: backlight 8008: light source 8009:frame 8010: printed circuit board 8011: battery

The invention claimed is:
 1. A display device comprising a pixel,wherein the pixel comprises a plurality of subpixels, wherein each ofthe subpixels comprises a display region, wherein the display devicecomprises, a first common electrode, a second common electrode, a pixelelectrode, an auxiliary wiring, and a liquid crystal layer, wherein thesecond common electrode overlaps with the first common electrode and thepixel electrode, wherein the second common electrode includes an openingin the display region of the subpixel, wherein in a cross-sectional viewof the display device, the opening of the second common electrodeoverlaps with a region which is not provided with the pixel electrode,wherein at least either of the first common electrode or the pixelelectrode does not overlap with the opening, wherein the auxiliarywiring is in contact with the first common electrode, wherein, when athickness of the liquid crystal layer is denoted by d, a width of theopening is greater than or equal to d/2 and narrower than a width of thesubpixel, wherein, when a thickness of the liquid crystal layer isdenoted by d, a distance between the openings of two adjacent subpixelsis greater than or equal to 1.2d and less than or equal to 2.4d, andwherein a thickness of the liquid crystal layer is greater than or equalto 1.5 μm and less than or equal to 3 μm.
 2. The display deviceaccording to claim 1, wherein the first common electrode is electricallyconnected to the second common electrode.
 3. The display deviceaccording to claim 1, wherein a liquid crystal included in the liquidcrystal layer has a negative dielectric anisotropy.
 4. The displaydevice according to claim 1, wherein the transistor includes asemiconductor layer, wherein the semiconductor layer includes an oxidesemiconductor in a channel formation region, and wherein the pixelelectrode is electrically connected to the transistor.
 5. The displaydevice according to claim 4, wherein the semiconductor layer includesindium, zinc, and one of aluminum, gallium, yttrium, and tin.
 6. Thedisplay device according to claim 1, further comprising a scan line anda signal line, wherein a direction in which the scan line extendsintersects with a direction in which the signal line extends, andwherein the plurality of subpixels exhibiting the same color are alignedin a direction intersecting with the direction in which the signal lineextends.
 7. A module comprising: the display device according to claim1; and a flexible printed circuit board or an integrated circuit.
 8. Anelectronic device comprising: the module according to claim 7; and anantenna, a battery, a housing, a camera, a speaker, a microphone, or acontrol button.
 9. The display device according to claim 1, wherein thefirst common electrode includes an opening in the display region of thesubpixel, and wherein in a cross-sectional view of the display device,the opening in the second common electrode overlaps with the opening inthe first common electrode.
 10. The display device according to claim 9,wherein in a cross-sectional view of the display device, the width ofthe opening in the second common electrode is smaller than a width ofthe opening in the first common electrode.
 11. The display deviceaccording to claim 1, wherein the display device further comprises alight-blocking layer, and wherein the auxiliary wiring overlaps with thelight-blocking layer.